Red Stele

Symptoms of red stele include severe stunting, occasionally followed by death of plants. Symptoms first appear on plants located in low, poorly drained parts of the field. Affected plants become stunted as older leaves die and are replaced by smaller, younger leaves with short petioles. Young lateral roots are often completely rotted. New crown roots die back from their tips, producing a symptom called "rat tail." Splitting affected roots reveals the red stele symptom (red coloration in the core of the root above the rotted end) from which the disease gets its name. Red stele can also cause a dark brown discoloration of the crown; this symptom is identical to that caused by other Phytophthora species that infect strawberry.

Most infections are limited to winter and early spring in California. Optimum conditions for disease development occur when the soil is water-saturated during cool weather. Under these conditions the pathogen produces zoospores (motile spores) that swim to the roots and infect them. Well-drained soil can minimize disease incidence and severity.

To minimize disease incidence and severity:

• Locate strawberry fields on well-drained soil.
• Plant annually with certified transplants.
• Fumigate the soil before planting.
• Use raised beds to provide optimum drainage.
• Avoid excessive or insufficient amounts of irrigation water.
• Consider using systemic fungicides.

There are no commercially available California strawberry cultivars with resistance to the pathogen that causes red stele.

• Use raised beds.
• Carefully manage drip irrigation.
• Plant in noninfested soils that have good drainage.
• Use clean plant stock.
• 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.

Control red stele with cultural controls in organically certified strawberries.

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 field history or environmental conditions suggest significant disease risk.