Annual Reports

1996—UC IPM Competitive Grants Program

Decision Support

The UC IPM Project will consider projects whose aim is the development and promotion of decision systems useful in crop production and pest management. General areas of such work might include but are not limited to development or improvement of sampling or detection methods, quantification of crop loss, development or improvement of damage thresholds or action levels, risk assessment, economic or other evaluation of IPM programs, development or evaluation of expert systems or other computer models.

Development of monitoring programs for pests and biological control agents involving sampling decision rules, control action thresholds, or improved methods for quantifying pest abundance or potential crop loss can significantly reduce the riskiness of pest control, reduce the number of pesticide applications, and improve adoption of new practices. Control action threshold research should be based on the relationship of pest occurrence or abundance to measurable crop loss in order to ensure cost-effective decisions.

High priority will be given to proposals that clearly define objectives; will produce programs that can be easily and directly used in the implementation of integrated pest management; will focus on a biological system that has had sufficient preliminary investigation to allow completion and validation of the program in a reasonable amount of time; and will involve principal investigators who have demonstrated experience in systems modeling or systems management.

Modeling or expert systems proposals must specify the intended audience for the model, clearly identify the model's utility to that audience, and present a well-formulated plan for making it available to potential users. A proposed model structure must be included. Authors are required to prepare a user's manual and technical documentation for all programs, and the proposal must discuss how this will be accomplished

New Projects Funded for 1996-97
Continuing Projects Funded for 1996-97
Progress Reports for Projects that Ended in 1995-96
Final Reports for Projects that Ended in 1995

New Decision Support Projects Funded for 1996-97

LYGUS THRESHOLDS FOR LYGUS Review and refinement of decision thresholds for lygus bugs on California blackeye beans. (Year 1 of 2; $13,259)

Principal Investigator: P.B. Goodell, UC IPM Project, Kearney Agricultural Center, Parlier

Objectives: Evaluate the effect of a range of lygus bug populations on yield and quality of CB46 blackeye beans.

Evaluate current decision thresholds relative to timing of infestation.

Diane Ullman demonstrates procedures for collecting greenhouse thrips for ELISA testing to detect the presence of tomato chlorotic spot virus.

A monitoring system for predicting tospovirus epidemics by sampling for infective thrips. (Year 1 of 3; $15,000) Principal Investigator: D.E. Ullman, Entomology, Davis Objectives: Adapt the existing enzyme-linked immunosorbent assay (NSs ELISA) technology for use in greenhouse grown crops. Determine an inexpensive and easy method for trapping thrips so they are adequately preserved for use in ELISA and determine how long the NSs protein can be detected after death of the insect on the trap.

Refine the existing ELISA method for use on large numbers of insects. Establish that the ELISA can detect naturally infected thrips under a field setting.

Determine whether infected thrips abundance is correlated with tomato chlorotic spot virus incidence in greenhouse grown crops.

Develop a forecasting system and test it in a greenhouse grown crop.

Development of sampling procedures for western flower thrips, Frankliniella occidentalis, in greenhouse flower production. (Year 1 of 3; $21,419)

Principal Investigators: M.P. Parrella, Entomology, Davis; B.C. Murphy, Entomology, Davis

Objectives: Determine the within- and between-plant distribution of western flower thrips on roses and chrysanthemums for common cultivars during the growing season and determine the optimal sample unit for estimating western flower thrips (WFT) abundance.

Develop and evaluate a practical, grower-oriented sampling procedure using sticky cards, plant samples, or a combination of the two for monitoring WFT to determine the need for control measures.

Estimate the relationship between WFT density on plants and the degree of feeding injury on foliage and flowers.

Verify effectiveness of the sampling procedures under commercial greenhouse conditions.

Monitoring for dry fruit mummies in stone fruit orchards may be used to predict need for brown rot sprays and eliminate unnecessary fungicide applications.

Using detection of latent infections of Monilinia fructicola in stone fruits in California to predict brown rot at harvest and postharvest and reduce fungicides. (Year 1 of 2; $16,400) Principal Investigator: T.J. Michailides, Plant Pathology, Davis Objectives: Determine if incidence of latent infections of peaches and nectarines by Monilinia fructicola can be a predictor of brown rot of fruit at harvest and postharvest.

Use the predictor to minimize the use of preharvest fungicide sprays of stone fruit for the control of brown rot at harvest and postharvest.

Use the predictor to minimize the use of postharvest fungicide treatment of stone fruit for the control of postharvest brown rot.

Continuing Decision Support Projects Funded for 1996-97

SILVERLEAF WHITEFLY IN SAN JOAQUIN VALLEY

Phenology, seasonal host selection, and biology of the silverleaf whitefly (bemisia argentifolii bellows & perring) in the San Joaquin Valley. (Year 3 of 3; $36,377)

Principal Investigators: C.G. Summers, Entomology, Davis/Kearney Agricultural Center, Parlier; R.L. Coviello, UC Cooperative Extension, Fresno County; M.J. Jiménez, UC Cooperative Extension, Tulare County

Objectives: Determine the seasonal phenology and population dynamics, the seasonal host plant acceptability, and the crop host selection sequence of the silverleaf whitefly on selected hosts in the San Joaquin Valley.

Determine the overwintering biology of silverleaf whitefly and the role of selected crop and noncrop hosts in over wintering survival.

Establish thresholds values for nymphal populations causing broccoli white stalk and irregular ripening of tomatoes in the San Joaquin Valley.

Summary of Progress: Although adults do not survive, silverleaf whitefly successfully overwinters in the San Joaquin Valley as eggs, second and third stage nymphs, and "red-eye pupa." Prickly lettuce, cheeseweed, annual sowthistle, and black mustard are the most important overwintering weeds. Eggs and nymphs were also found on spotted spurge and black nightshade. These summer annuals frequently persist throughout the year in protected areas and contribute to overwintering survival. Cole crops constitute the majority of overwintering crop plants although avocado and orange also contribute to winter survival. Destruction of overwintering hosts is important in reducing whitefly numbers the following summer.

Spring 1995 was very different from that of 1994 with above normal precipitation and below normal temperatures. These conditions delayed buildup of silverleaf whitefly populations and contributed to significantly reduced infestation levels in late summer; numbers reached less than half those observed in 1994. This shows the importance of spring and early summer weather conditions in the phenology of silverleaf whitefly.

Host acceptance in warm season crops in 1995 was the same as in 1994. In descending order of host acceptance were melons, pima cotton, acala cotton, tomatoes, blackeye beans, alfalfa, sugarbeets. In processing tomatoes, as whitefly populations decreased, yields of ripe fruit increased. As whitefly populations declined, the yield of irregular ripened fruit declined. Results for 1995 were similar to 1994 indicating that very low levels of whitefly (less than 25 per leaf) can cause substantial, irregular ripening.

Maturity is delayed by increasing whitefly density; a significantly higher percentage of green fruit (and lower percentage of ripe fruit) was harvested from high whitefly density plots. A delay in maturity caused by silverleaf whitefly has not been previously documented. Similar results were observed in fresh market tomatoes. Increasing silverleaf whitefly densities resulted in a higher percentage of irregular ripening. At very low whitefly densities (less than 25 per leaf), irregular ripening still averaged about 20%. Very low densities of silverleaf whitefly also caused white stock in broccoli. At high whitefly densities, broccoli stocks were almost "pure white" in color.

Integrated management of aphids and aphid-vectored virus. (Year 3 of 3; $26,026)

Principal Investigators: J.J. Stapleton, UC IPM Project, Kearney Agricultural Center, Parlier; C.G. Summers, Entomology, Davis

Objectives: Determine and compare the effectiveness of combinations of reflectorized plant coatings, soil mulches, and the aphid alarm pheromone in delaying and/or reducing the development of aphid-borne virus diseases and subsequent crop loss in susceptible vegetable varieties.

Determine the extent to which these various methods and combinations repel aphids from plants thus delaying and/or reducing colonization and virus transmission.

Determine the effects of these treatments on earliness, health, yield, and economics of susceptible vegetable crops.

Summary of Progress: A complex of aphid-vectored virus diseases is causing increasingly heavy losses in many vegetable crops grown in the inland valleys of California. Until multiple virus resistance can be developed for the numerous commercial cultivars of vegetable crops, economic losses are likely to continue or increase.

Three field experiments were conducted in 1995 to test the effectiveness of reflectorized, spray, and polyethylene soil mulches, plant coatings, and the aphid alarm pheromone, alone and in combination, for management of aphids and aphid-borne virus diseases in San Joaquin Valley vegetable crops. One experiment each was done using cantaloupe melons, zucchini squash, and fresh market tomato as test crops.

Excellent yield responses from melon and zucchini were obtained to the reflectorized mulches under severe virus conditions. Aphid numbers on mulched plants were consistently lower than on nonmulched plants in all three experiments. Onset of symptoms of cucumber mosaic, watermelon mosaic, and zucchini yellows mosaic viruses were delayed 3 to 6 weeks in the cucurbitaceous crops, and cucumber mosaic, tobacco etch, and tomato ringspot viruses for a similar period in the tomato crop, which was critical for facilitating normal flowering and fruiting. The high virus disease incidence reduced cumulative marketable squash and melon yields (both number and fresh weight) to near zero in the control plots. Both spray and polyethylene mulch treatments provided large differences in yield of zucchini squash and cantaloupe melon.

For example, all mulch treatments gave large increases in marketable melon yield over the nonmulched controls, ranging from 16.8-fold (50% bed-width mulched) to 25.8-fold (polyethylene 100% bed-width mulched). Although results with tomato were less conclusive due to frost prior to harvest, preliminary assays of vegetative growth showed increased growth in mulched plots. No significant benefit from plant coatings or the aphid alarm pheromone was obtained on any of the crops.

Research on rice-weed competition showed that good stand establishment negates the need for chemical control of California arrowhead, Sagittaria montevidensis.

Management-oriented simulation of rice growth and weed competition. (Year 3 of 3; $26,585) Principal Investigators: T.C. Foin, Environmental Studies, Davis; J.E. Hill, Agronomy and Range Science, Davis Objectives: Continue experimental research on rice-weed interactions that (1) advances the search for the characteristic properties that are relatively independent of the identity of the weed species; and (2) provides the parameter values necessary for the simulation models.

Complete the revisions to the California Rice (CARICE) simulation model needed to adapt it for predicting weed impacts and to evaluate alternative strategies for weed control.

Summary of Progress: We have done field and greenhouse experiments on rice competition for light that included combinations of the following weed species: Ammannia spp. (redstem), Cyperus difformis (smallflower umbrellaplant), Echinochloa spp. (watergrasses), Sagittaria montevidensis (California arrowhead), and Scirpus mucronatus (ricefield bulrush). The most important findings are:

1. Weeds reduce rice tiller density through at least two shading mechanisms: early-season suppression of tiller formation, and increased tiller mortality after maximum tillering. Fast-growing weeds with large leaf areas late in the season may affect both processes. Documenting these mechanisms has helped to explain weed competitive ability and may allow us to optimize weed control timing.

2. Many important rice-field weeds (e.g., Ammannia spp., Echinochloa spp.) adjust their growth rates to stand conditions, increasing both weed establishment and weed seed production. Weeds probably respond plastically to some management practices (e.g., plant density), and this should be considered when developing IPM programs.

3. The 1995 field experiments showed that delaying herbicide application to take full advantage of rice-canopy closure maximized weed control through canopy suppression of weed regrowth. Total broadleaf and sedge weed density was reduced 67% at final harvest by application 15 days after seeding (DAS), 4 to 6 days later than normally practiced by this grower. This experiment will be repeated in 1996 to compare very early (less than 7 DAS) and later (~14 DAS) application dates.

4. The field experiment also reaffirmed an earlier finding that S. montevidensis is suppressed by dense rice stands. It had the highest weed density and drymass at mid-season harvest (46 DAS), yet was completely absent at the final harvest (126 DAS). Because of its height disadvantage, S. montevidensis cannot compete over the full season. Good stand establishment negates the need for chemical control of S. montevidensis.

Risk assessment for fruit scarring by citrus thrips, Scirtothrips citri (Moulton), from soil and air temperatures before petal fall. (Year 2 of 2; $5,862)

Principal Investigators: H. Schweizer, Entomology, Riverside; J.G. Morse, Entomology, Riverside

Objective: Develop a temperature-driven, phenological model for estimating the amount of fruit scarred by citrus thrips, which can be used to decide whether or not control measures are needed.

Summary of Progress: Historical (1982-93) citrus thrips scarring data from one 'Navel' orange plot has been analyzed in relation to temperature patterns. 1991 was excluded from the analysis; it was a season after a severe frost, which might have altered tree physiology or populations of natural enemies of citrus thrips. We have been able to explain around 65% of the inter-year variability in severe thrips scarring with a regression model taking into account degree-day accumulations before petal fall. In 1991, after the freeze, thrips scarring was much more severe than would have been predicted by the regression model, suggesting that citrus thrips populations must have done exceptionally well, or the newly set fruit must have been exceptionally susceptible to scarring by thrips.

In spring 1995, ground and air temperatures, citrus thrips populations, and tree phenology were monitored at a total of 11 'Navel' orange sites in the San Joaquin Valley (from Woodlake to 48 km south of Porterville). In early summer 1995, the weather was exceptionally cool and citrus thrips scarring was very low (less than 3% severe scars on outside fruit at all sites). We were unable to detect any relationship between temperatures at the different sites and different citrus thrips scarring at these sites, perhaps due to the very cool season. The first generation adult population, percent fruit infested with thrips larvae, and numbers of larvae found per fruit, were relatively well correlated with fruit scarring.

The results confirm that the currently used sampling method, fruit infestation, is a valuable measure for predicting thrips scarring. However, the first adult generation can be monitored earlier and thus would provide earlier information upon which to base fruit scarring estimates. Presently, our data suggest that temperature conditions before petal fall in conjunction with adult samples of the first generation might provide a means of estimating the risk of citrus thrips damage earlier than examining fruit for thrips infestation after petal fall.

Progress Reports for Decision Support Projects that Ended in 1995-96

GRAY MOLD PROTECTION FOR KIWI

Using levels of kiwifruit sepals and stem ends colonized by Botrytis cinerea to predict gray mold and reduce fungicides.

Principal Investigators: T.J. Michailides, Plant Pathology, Davis; D.P. Morgan, Plant Pathology, Davis

Objectives: Determine if incidence of colonization of kiwifruit sepals and stem ends by Botrytis cinerea in vineyards can be a predictor of Botrytis gray mold decay of fruit in storage.

Use the predictor to minimize the use of preharvest vinclozolin (Ronilan) sprays for controlling Botrytis gray mold.

Summary of Progress: Gray mold storage decay caused by B. cinerea is the most important disease of kiwifruit, even though B. cinerea is not considered a field pathogen in California kiwifruit because the disease does not occur in the field. Postharvest decay by gray mold, however, is commonly responsible for large losses during long-term storage of kiwifruit and storage decay is a direct result of B. cinerea infections that do occur in the field but remain latent in the senescent floral parts (mainly sepals) and stem end scars (receptacles).

In this project, it was possible to successfully predict gray mold decay of kiwifruit in storage from the incidence of colonization of sepals and stem ends by B. cinerea. In 1994 the incidence of sepal colonization decreased 3 months after fruit set and then increased until harvest time. In contrast, the incidence of receptacle colonization by B. cinerea increased continuously from 1 month after fruit set until harvest.

Preliminary results from 1992 and results in both 1993 and 1994 showed that the levels (incidences) of colonization of sepals and receptacles is a good predictor of the gray mold in storage. In 1994 all regressions were significant and the best correlation was obtained with samplings done 4 months after fruit set. In both 1993 and 1994, low (less than 15%), medium (16 to 50%), and high (more than 50%) colonization levels of sepals or receptacles by B. cinerea predicted the majority of the vineyards as having low (less than 2%), moderate (2 to 6%), and high (more than 6%) incidence of postharvest gray mold, respectively, after 5-month storage. Similarly, the 1994 incidence of B. cinerea colonization of sepals and receptacles predicted correctly the incidence of gray mold in seven out of eight and six out of eight vineyards, respectively, after 3-month storage.

In vineyards where the incidence of gray mold decay was low (less than 2%), one or two preharvest sprays of vinclozolin did not reduce significantly the incidence of gray mold. Therefore, no spray was needed in these vineyards. However, preharvest spray(s) of vinclozolin applied 1 and/or 2 weeks before harvest in vineyards with lmore than 6% anticipated (predicted) gray mold significantly reduced fruit decay in storage. Thus, the spray(s) was(were) needed in these vineyards.

Although in 1993 and 1994 some of the vineyards had fruit with <15% incidence of colonization of sepals or receptacles by B. cinerea, in 1995 none of the vineyards had <15% incidence of colonization by B. cinerea. Perhaps the very wet spring in 1995 contributed to higher levels of latent infection by B. cinerea. But still we could distinguish the vineyards by three categories as follows: with low (up to 25%), medium (26-50%), and high (>50%) incidence of colonization of sepals or receptacles of fruit sampled 3 and 4 months after fruit set. The vineyards fell into the three categories of B. cinerea colonization as expected, based on the history of gray mold of their fruit kept in storage, although in general, the levels of colonization were higher in 1995.

As stated earlier, all experiments were carried out as planned and fruit from treated and nontreated vines have been stored, a waiting evaluation in January and again in mid-March (after 3- and 5-month storage, respectively). The preliminary experiments in 1992 and results in 1993 and 1994 suggest that we have developed a successful method to predict gray mold in storage, findings on which this proposal was based. Presently, two refereed articles are in press in California Agriculture and Plant Disease.

Researchers developed economic thresholds and sampling methods to determine when crop rotation is justified against sugarbeet cyst nematode in northern California. Shown here are the white mature female and the brown cyst containing eggs of the sugarbeet cyst nematode on a sugarbeet root.

Sequential decision methods to control sugarbeet cyst nematode via rotations. Principal Investigators: B.B. Westerdahl, Nematology, Davis; O.R. Burt, Agricultural Economics, Davis; E.P. Caswell-Chen, Nematology, Davis Objectives: Establish a damage threshold for sugarbeet cyst nematode on sugarbeets in northern California. Refine and extend the theory and methods of critical point (damage threshold) models for controlling nematodes by means of cropping sequences.

Summary of Progress: Approximately 228,000 acres of sugarbeets distributed among 29 counties are grown each year in California. The cash farm income reported in 1987 for sugarbeets was $213 million. In 1978, 366,565 acres in California were reported to be infested with the sugarbeet cyst nematode (SBCN) (Heterodera schachtii).

A 3- to 4-year rotation to nonhost crops is the currently recommended management program for SBCN on sugarbeets. This crop rotation program is widely practiced in the Imperial Valley where profitable rotation crops are available. On the other hand, growers in infested areas of northern California are in need of improved management programs because of the lower profitability of all available rotation crops (typically beans, wheat and corn), except tomatoes, makes them less feasible.

Northern California sugarbeet growers cannot reliably practice crop rotation without (1) a knowledge of the rate of population decline under nonhost crops, (2) a damage threshold for the two most commonly practiced planting regimes (late spring planting with overwintering of beets followed by spring harvest, or early spring planting followed by fall harvest), and (3) improvements in the economic criteria for choosing an optimal sequence of crops.

Repeated sampling of several fields over a period of 6 years, indicates that population decreases of 20 to 25% per year are typical for northern California. Development of damage thresholds and improvements in economic criteria for planning crop rotations are the goals of this project. Field and microplot experiments are in progress to develop a damage threshold for different planting dates. The information gathered to date indicates that the damage threshold is approximately 1 egg/gram of soil or less and that we are able to measure levels as low as 0.04 eggs/gram of soil.

An economic model has been developed to determine the SBCN population density at which sugarbeets should be planted again after a sequence of nonhost crops and will be tested and refined utilizing the data obtained from field and microplot experiments.

Implementation of a disease warning system for downy mildew of lettuce using high resolution weather forecasts and a geographical information system.

Principal Investigator: A.H.C. van Bruggen, Plant Pathology, Davis

Objectives: Couple an existing infection model for lettuce downy mildew with high-resolution weather forecasts and with an existing simulation model for leaf wetness duration to generate field-specific predictions of daily infection risk, and validate these predictions against field data at selected locations in the coastal valleys.

Develop a Geographical Information System (GIS), incorporating data layers for topography, inoculum availability, and weather and leaf wetness forecasts, to calculate daily infection risk for any location in the coastal valleys.

Develop methods to automate the input of weather forecasts and up-to-date information about downy mildew inoculum availability into the GIS, and implement the GIS-based disease warning system using UC IPM and UCCE resources.

Summary of Progress: The overall objective of this research is to develop a warning system for improved chemical control of lettuce downy mildew based on weather forecasts. The use of forecast weather, rather than measured (past) weather, could provide sufficient lead time for growers to schedule and perform spray applications before infection takes place, thus improving the efficacy of protective fungicides.

Our previous studies in commercial lettuce fields in coastal California had shown that infection with downy mildew occurs mainly during periods of prolonged morning leaf wetness duration (LWD), suggesting that fungicide applications could be timed according to morning-LWD.

A simulation model for LWD caused by dew was used in conjunction with medium-resolution weather forecasts (obtained from Skybit Inc.) to predict prolonged morning-LWD periods (and thus disease risk) with a lead time of 24 hours. These leaf wetness predictions were compared with local leaf wetness forecasts provided by two weather forecasting companies (Fox Weather and Skybit Inc.) and measured leaf wetness at three locations. Predicted temperatures and relative humidities were also compared with observed values. Temperature predictions were generally in agreement with predicted values, but relative humidity predictions were often underestimated. Similarly, the number of hours with leaf wetness was underestimated by the weather forecasting companies, while the model predictions missed fewer leaf wetness periods than the commercial forecasters.

In field trials, too many mornings with prolonged leaf wetness (including those caused by drizzle) were missed so that disease severity was higher when sprays were applied according to the forecasting system than when sprays were applied on a calendar-based schedule. In the implementation phase of the forecasting system, modifications will be made so that drizzle will be included in the forecasts, the threshold between wet and dry for the leaf wetness sensors will be lowered, and spore survival during overcast days and subsequent infection at night will be incorporated. Moreover, disease warnings will be issued based on a combination of predicted and observed weather and leafwetness rather than on predicted leafwetness only.

Considerable progress was made with regionalization of the disease warning system. As a first step, results of field surveys on downy mildew incidence and severity were entered and analyzed in GIS. Temperature data from CIMIS stations in the Salinas area were entered into GIS, in combination with a digital elevation map of the region. This is a good start in the development of regional disease warning systems based on local infection risk derived from observed and predicted weather and regional disease severity.

Research on leafminer outbreaks in lettuce showed that cauliflower was an important overwintering host for the pest in the Salinas Valley.

Study of Liriomyza leafminer outbreaks in lettuce. Principal Investigators: J. Granett, Entomology, Davis; W. Chaney, UC Cooperative Extension, Monterey County Objectives: Identify sources of Liriomyza infestations in California lettuce production areas. Distinguish relative contributions of specific mortality factors to the overall mortality of Liriomyza in the field. Summary of Progress: Liriomyza leafminer infestations are a problem on lettuce and other crops in the Salinas Valley area. There are almost as many sources of these flies as there are crops grown in the Salinas Valley, but the importance of each source changes through the year. Head lettuce is grown from late January through November in the Salinas Valley, but both adult and larval flies are present and active throughout the year. We found no evidence that the flies overwinter as puparia in fallow fields. Broccoli and cauliflower serve as hosts (and perhaps as reservoirs for the flies) when lettuce is not available.

In one study, an aging, seed broccoli field served as a significant source of colonizing flies for a February-planted head lettuce field. In a second early season head lettuce study field surrounded by fields of broccoli, lettuce, and rye, there was no dominant direction of source-flies. As the year progresses, sources of flies can be found in any lettuce, celery, cauliflower, broccoli, spinach, green onion, mustard, and vetch field (and in numerous weeds and ornamentals), but we have found lettuce to be the greatest relative producer of leafminers.

Management of the sources of leafminer infestations might best be practiced during the winter months (December and January) when host crops are limited. Currently, broccoli and cauliflower are not managed for leafminers, but clearly harbor winter populations which serve as sources for later infestations.

Several mortality agents exert a toll upon leafminer populations. We observed high levels of egg mortality in the field (40 to 70%), and see evidence that a plant defense is responsible for most of this mortality. No predation, and negligible parasitism were found. An application of a fungicide resulted in some mortality of the larval leafminers.

Development and use of molecular markers for the control of lettuce downy mildew.

Principal Investigators: R. Michelmore, Vegetable Crops, Davis; K. Subbarao, Plant Pathology, Davis; S. Koike, UC Cooperative Extension, Monterey County; F. Laemmlen, UC Cooperative Extension, Santa Barbara County

Objectives: Use reliable DNA fingerprints from PCR-based, molecular markers for lettuce downy mildew to monitor downy mildew populations in California.

Characterize patterns of spread through lettuce growing regions.

Determine the amount of variation within commercial fields.

Provide information for the optimal use of resistant cultivars and fungicide treatments.

Summary of Progress: We collected isolates of the fungus that causes lettuce downy mildew on a monthly basis from throughout the Salinas and Santa Maria growing areas during the growing season as well as receiving isolates from cooperating pest control advisers. We characterized a total of 367 isolates for sensitivity to the fungicide, Ridomil, and a subset for their ability to overcome plant resistance genes and their mating type. Approximately 20% of isolates were sensitive to Ridomil, 15% had intermediate sensitivity, and the remainder were insensitive. There was no correlation with region or time in the season. In contrast to surveys before 1990, isolates varied widely in their ability to overcome resistance genes.

The data indicates that the pathogen population is in transition from one that was dominated by a few asexual types to a sexually-reproducing population that is much more variable and will likely be much more difficult to control. Both of the two mating types were identified, although one predominated. None of the isolates of the rare mating type were Ridomil insensitive; this provides the opportunity to slow the transition to a highly variable sexual population through targeted fungicide applications. We are continuing to develop DNA fingerprinting techniques to allow a greater number of isolates to be characterized quickly so that fungicide applications can be made on an effective timescale.

The incidence of tomato infectious chlorosis virus in greenhouse and field tomatoes and management strategies for its control.

Principal Investigators: B.W. Falk, Plant Pathology, Davis; S. Temple, Agronomy and Range Science, Davis; R.M. Davis, Plant Pathology, Davis

Objectives: Perform a survey of the incidence of tomato infectious chlorosis virus (TICV) in California tomatoes, specifically north of the Tehachapi's.

Summary of Progress: Specific methodologies were perfected for detecting TICV in plants. We used complementary DNA (cDNA) probes generated by us previously, as well as some new probes constructed during this work. A non-radioactive assay was used to positively and accurately detect TICV in fresh or frozen leaf extracts. A survey for the incidence of TICV was done by collecting samples from several areas, both from greenhouse and field grown plants, and analyzing for TICV in the laboratory at UC Davis. A total of 396 samples were collected from Yolo, San Joaquin, Stanislaus and Monterey counties. Of these, only 57 tomato plants were found to be infected by TICV. Most were older greenhouse-grown plants, but included samples from Yolo and San Joaquin counties. The only TICV-infected field tomatoes were old plants that originated as transplants. The transplants were originally grown in a greenhouse that also contained older TICV-infected tomato plants being grown for seed production.

Final Reports for Decision Support Projects that Ended in 1995

IDENTIFYING NEMATODES WITH RAPD

Use of random amplified DNA polymorphisms for root knot nematode biotype identification.

Principal Investigators: V. Williamson, Nematology, Davis; E.P. Caswell-Chen, Nematology, Davis; B.B. Westerdahl, Nematology, Davis

Summary of Accomplishments: Root knot nematodes (RKNs) are the major nematode pests in California field and vegetable crops. There are several RKN species and biotypes that differ in the plant hosts that they parasitize. Random amplified polymorphic DNA (RAPD) markers that distinguish Meloidogyne hapla and M. chitwoodi from each other and from other root knot nematode species were identified. Primers that specifically amplified a DNA fragment from each species were developed. Successful amplifications from single juveniles were readily attained. A mixture of two pairs of primers in a single PCR reaction mixture was shown to identify and distinguish single juveniles of M. hapla and M. chitwoodi. The primers allowed successful amplification of DNA from M. hapla isolates from several different crops and locations in North America and isolates of M. chitwoodi that differ in host range confirmed primer specificity.

We also developed species-specific primers that allow discrimination of M. incognita, M. javanica, M. arenaria, and M. hapla using PCR with multiple primers in one reaction; however, discriminating these species requires multiple DNA amplification procedures. We are still assessing how robust these primers are for root knot nematodes from different geographic isolates.

This new assay is useful as it can be used to identify the species of individual juveniles and, thus, does not require that adult female RKN be present. This has allowed us to identify the species in diagnostic samples where only juveniles are present. Previously, we were able to identify the species precisely only when adults were present.

Improved monitoring methods for citrus cutworm.

Principal Investigator: E.E. Grafton-Cardwell, Entomology, Riverside/Kearney Agricultural Center, Parlier

Summary of Accomplishments: This project has improved integrated pest management of citrus cutworm by providing the grower or pest control adviser with methods to monitor for and predict citrus cutworm development in the field. A single pheromone trap in a 10- to 20-acre citrus orchard can provide the grower with a biofix for when citrus cutworm moths first begin flying (more than one moth in 2 consecutive weeks) and an indication of the severity and duration of egg laying and larval emergence.

When this information is combined with degree-day (DD) monitoring (lower developmental threshold of 49.1° F), growers can estimate that larval emergence will begin approximately 236 DD after the biofix date. At that point in time, growers can monitor for citrus cutworm larvae using a time search, sweep net, or beating sheet method. Peak larval activity usually occurs 2 to 4 weeks after the larvae begin emerging.

If the number of larvae exceeds the economic threshold before petal fall then soft pesticides (Bacillus thuringiensis products) that do not kill natural enemies can be used to reduce the citrus cutworm population. Timing of the Bacillus thuringiensis pesticide applications is especially important because they have a short residual action and are most effective in killing the smaller instars of larvae. Economic thresholds before petal fall are 10 to 15 worms per hour search, 10 to 15 worms per 25 net shakes, or 18 to 22 worms per 25 beating sheet shakes. If the larval emergence is delayed or prolonged and occurs after petal fall, the threshold drops to three to five worms per method and the quicker acting broad-spectrum pesticides are recommended for control. Economic densities of citrus cutworm larvae will not always cause significant fruit scarring, but may instead lower yield. If the economic threshold is not reached, growers can avoid spraying pesticides that are not needed.

A 3-year field study of rice water weevil impact on rice yields established economically damaging population levels; drill seeding reduced larval densities over water seeding.

Influence of rice seeding method and rice variety on damage potential and economic threshold of rice water weevil. Principal Investigators: A.T. Palrang, Entomology, Davis; L.D. Godfrey, Entomology, Davis Summary of Accomplishments: A 3-year field study was conducted to determine the effects of rice water weevil (RWW) infestation on rice plant growth, development, photosynthesis, and yield. Studies were conducted in water-seeded 'M-202' rice (1993-95), drill-seeded 'M-202' (1993-94), and water-seeded PI line (1993-94) under development with RWW tolerance. Differential larval densities were established by infesting plots with 0 to 0.6 RWW adults per plant.

Data on adult feeding scars on rice leaves were consistent over years, seeding method, and variety. In the uninfested plots, the percentage scarred plants averaged ~5-8% (a low background level of infestation existed) compared with 70 to 80% in the highest infestation level (0.6 adults per plant).

In 1993 and 1995, rice water weevil larval densities, the damaging stage, correlated closely with adult infestation levels in the 'M-202' water-seeded treatment and the average larval density increased from 1 to 17 per plant as the adult infestation density increased. In 1994, treatments generally resulted in uninfested plots and in infested plots (averaging ~8.3 larvae per plant). Larval density was slightly reduced in the water-seeded PI line and significantly reduced to a maximum of 3.8 larvae per plant in the drill-seeded 'M-202' compared with the water-seeded 'M-202'.

Reductions in rice plant growth were seen at infestation levels equal to or greater than 0.4 adults per plant (=~7 larvae per plant). The number of tillers per plant, and resulting leaf area per plant, was most severely reduced by rice water weevil larval injury. Rice plant maturity was not altered by larval injury. Rice photosynthetic rate, the energy mechanism of the plant, was reduced by the root injury during the period of larval feeding; however, photosynthetic rates of injured plants elevated to that of uninjured plants at and following the time of RWW pupation. Grain yield was reduced by up to 45% in 1993 and by 33% in 1995 by the highest infestation levels. Over the 3 years, the yield loss averaged 1.6% per RWW larva.