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Project description

Evaluation of Control Practiced for Peach Yellow Leafroll Disease. (94BU023)
Program UC IPM competitive research grants program
B.C. Kirkpatrick, Plant Pathology, UC Davis
Host/habitat Stone Fruits; Tree Crops; Peaches
Pest Peach Yellow Leafroll
Discipline Plant Pathology
Biorational Use of Biotic Agents or Chemicals
Start year (duration)  1994 (Three Years)
Objectives Determine if premature defoliation or late season insecticide applications can decrease the incidence of peach yellow leafroll (PYLR) disease in four high risk peach orchards located in Northern California.

Determine what effect, if any, zinc sulfate mediated defoliation has on peach yields under Northern California growing conditions.

Monitor and determine the infectivity of mycoplasma-like organism (MLO) insect vectors caught in yellow sticky traps placed in the four high risk orchards.

Determine what PYLR-MLO strains are causing new infections in the 4 test orchards and other orchards that are currently using pheromone disruption practices.

Molecular diagnostic assays performed on samples collected in 1993 established that all of the peach trees in our four high risk PYLR test orchards were infected with a MLO that also infects pear trees and is transmitted by pear psylla. Because pear psylla only migrate out of pear into peach late in the fall, we hypothesized that PYLR infections could be reduced by eliminating or protecting peach tissues (primarily leaves) that MLO-infectious psylla feed upon late in the fall. We are investigating whether premature defoliation, accomplished by foliar applications of zinc sulfate, or pyrethroid insecticides can reduce losses to PYLR disease.

In 1994 it was shown that blossom set and fruit yields on trees prematurely defoliated with zinc sulfate in October, 1993 were indistinguishable from untreated controls. This result suggests there is no immediate deleterious effects associated with fall applications of zinc. However, it will be necessary to repeat these treatments on the same trees to insure that there are no potential cumulative effects associated with zinc-mediated defoliation.

In 1993 and 1994 we mapped the locations of PYLR-infected peach trees in four high risk, northern California orchards and established randomized blocks of the following four spray treatments in each test orchard: water control, zinc sulfate-mediated defoliation, pyrethrin (Asana) insecticide only, and zinc with insecticide. All materials were applied by grower cooperators using a conventional airblast orchard sprayer during the first week of November, 1994. The efficacy of these treatments will be established by determining the number of new PYLR infections in each treatment block in 1995. Samples will be taken from newly infected PYLR trees and the type of MLO causing the new strike will be determined. In this manner we will establish whether any of our treatments are effective in reducing PYLR tree losses.

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