Phytophthora Crown and Root Rot

Initially, symptoms typically include plant stunting and small leaves. As the season progresses, plant collapse may occur rapidly or slowly. When infected plants are cut open, a brown discoloration can be seen in the crown vascular tissue or throughout the crown tissue. Infection of the roots causes a brown to black root rot.

Of the Phytophthora species involved, P. cactorum is the most common; the others are much less prevalent on strawberry. Phytophthora is soilborne. When the soil becomes saturated with water, the pathogen can produce and release zoospores, which swim through water-filled soil pores to infect plant tissue. Phytophthora species also produce resilient spores (chlamydospores, oospores) that enable them to survive in soil for long periods without a host or under adverse conditions. Infections can occur during cool to moderate temperatures, which are typical throughout coastal fruit-production cycles.

Soil fumigation and good cultural practices provide adequate control of Phytophthora in production fields. Good cultural practices include the use of certified transplants, avoiding poorly drained soils, and preparing fields to provide good soil drainage during wet weather. Phytophthora can be moved in water that has drained from infested fields, so avoid using runoff water for irrigation or for wetting down field roads for dust control. Plant less-susceptible cultivars, such as Fronteras, Merced, Albion, San Andreas, and Portola. Even with tolerant cultivars, however, it is important to follow good cultural practices.

• Use raised beds and carefully manage drip irrigation; plant in noninfested soils that have good drainage.
• Use clean plant stock and consult your farm advisor about cultivar susceptibility.
• For non-coastal, warmer regions consider soil solarization.

In warmer areas of the state, solarization has been shown to be effective for the control of soilborne pathogens and weeds. Solarization is carried out after the beds are formed and can be effective if weather conditions are ideal (30-45 days of hot weather that promotes soil temperatures of at least 122°F). The effectiveness of solarization can be increased by solarizing after incorporating the residue of a cruciferous crop, in particular broccoli or mustards, into the soil or following an application of metam sodium (40 gal/acre). For more details on how to effectively solarize soil, see Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds, UC ANR Publication 21377.

Use good cultural practices such as pathogen-free planting stock, resistant or tolerant varieties and careful water and soil management (improving soil drainage; raised beds).

If drip fumigation is planned, good results have been obtained with a sequential application of chloropicrin or 1,3-dichloropropene plus chloropicrin followed 7 days later with metam sodium or metam potassium. Preplant dips and foliar sprays with fosetyl-aluminum or postplant ground or drip applications of mefenoxam are advisable when Phytophthora-susceptible cultivars are used or when field history or environmental conditions suggest significant disease risk.