Phytophthora crown and root rot of grape—Phytophthora cinnamomi
Various Phytophthora species cause decay of the basal stem (crown) and roots of numerous annual and perennial plants. In many hosts Phytophthora diseases are seriously damaging or lethal.
Several species of these oomycete (water mold) pathogens have been isolated from grapes in California. However only P. cinnamomi has been found to cause disease in California grapes, and in grapes the disease is generally a minor problem.
Identification
Grapevines affected by Phytophthora crown and root rot may be undersized and have sparse foliage that drops prematurely. In red-fruited varieties leaves may turn red in summer or drop prematurely due to the pathogen's girdling of the basal trunk and root crown. Vines with these symptoms may die during the winter or the following growing season.
Lookalikes. Armillaria root rot and Pierce's disease can cause the same symptoms as above. However, when diseased by Armillaria, removing bark from the root crown commonly reveals whitish mycelia of the fungus. Phytophthora species produce no obvious mycelia.
When Pierce's disease is the cause of unhealthy grapevines, four symptoms in mid- to late summer indicate its presence:
- Leaves become slightly yellow or red along margins in white and red varieties, respectively. Eventually leaf margins dry or die in concentric zones.
- Dried leaves drop prematurely, leaving the petiole (leaf stem) attached to the cane, a symptom called "matchsticks."
- Fruit clusters shrivel and berries become raisinlike.
- Wood on new canes matures irregularly, producing patches of green surrounded by mature brown bark, a symptom called "green islands."
All four symptoms are not necessarily present when Pierce's disease is present. When in doubt about the presence of Pierce's disease, samples of vines can be sent to a plant diagnostic laboratory to determine whether they are infected with the Xylella fastidiosa bacterium that causes Pierce's disease.
To diagnose Phytophthora crown and root rot, remove soil around the root crown. Cut off some outer bark at and just below the soil line. Instead of healthy green cambial tissue and whitish wood, crown tissue diseased from Phytophthora turns blackish to dark brown. Instead of abundant feeder (small) roots, the grapevines will have few small roots. Removing bark from small roots or pulling them apart may reveal internal tissue that is dark and decayed instead of firm and whitish as with healthy rootlets.
Life cycle
Certain spore types of Phytophthora can persist in soil for many years in the absence of a living host. When there are frequent, prolonged periods of soil being saturated with water (waterlogging e.g., soggy for more than 24 hours), this stimulates the germination of oospores that produce sporangia that in turn produce zoospores. This production of zoospores occurs in poorly (slowly) draining soil or after heavy rain or irrigation practices that cause frequently soggy soil. The zoospores swim through water-filled pores in soil. They are attracted by root exudates and infect the roots.
Long distance spread of this pathogen can occur from planting stock that was infected in the nursery. Anything that results in the movement of soil or surface water during flooding or runoff can spread Phytophthora. Many garden and landscape plants are hosts of P. cinnamomi. Where such hosts are infected and growing nearby, they can be the source of Phytophthora infection of grape.
Damage
Phytophthora crown and root rot is a minor disease of grapes in California. It occurs mostly where waterlogging is a problem in fine-textured soils in coastal valleys. When vines become diseased and their crown is completely girdled by the pathogen, affected vines die quickly or within about 1 year of initially showing symptoms.
Solutions
Controls for Phytophthora crown and root rot are primarily preventive. Obtain planting stock from a reputable, commercial supplier that sells disease-free plants. Where soil drains poorly, avoid the rootstocks Kober 5BB and Rupestris St. George because these are more susceptible to P. cinnamomi. Plant the grapevines on a raised berm or mound of soil to improve drainage. Keep the graft union of vine trunks above the soil line.
Carefully manage irrigation to avoid prolonged soggy soil. Use drip or furrow irrigation instead of sprinklers. If using sprinklers, position nozzles to prevent water from directly wetting crowns and trunks. Use sophisticated methods to estimate the amount and frequency of needed irrigation, such as according to local evapotranspiration (ET) rates or the use of soil moisture monitoring devices such as tensiometers.
Fungicides (oomyceticides) may not be highly effective at controlling Phytophthora crown and root rot in gardens and landscapes. Where the appropriate cultural practices are employed, to help manage Phytophthora infection, fosetyl-al may be sprayed on foliage or phosphorus acid may be applied to foliage or soil. Do not rely on fungicide applications alone to provide control as the results can be unsatisfactory.
For more information, see Pest Notes: Phytophthora Root and Crown Rot in the Garden. Adapted from the publication, Grape Pest Management Third Edition from UC Agriculture and Natural Resources also available as an eBook and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM). |
