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Strawberry (Fragaria spp.) - Crown Rot
Cause The fungal-like organism Phytophthora cactorum causes a subtle fruit rot of strawberry that imparts off flavors and smells in both fresh and processed berries. It appears to be different from the same fungus that causes crown rot. Oospores form in mummified fruit. They can also survive long periods in the soil. In the spring under saturated wet conditions, new spores are transferred to fruit by rain splash, overhead irrigation, or free-swimming zoospores. Fruit can be infected at any stage of development. New spores are produced in as little as 5 days after infection under warm (greater than 50°F), wet conditions.
Symptoms Green fruit will have areas of dark-brown lesions that eventually spreads to the whole berry. The texture of the infected berry appears leathery. Mature fruit may turn brown to dark-purple or may not change in color much at all. The vascular tissue to each seed is darkened. In advanced stages of decay the fruit becomes tough and leathery. All infected fruits dry down to a hard mummy. Infected fruit has a diagnostic unpleasant odor and taste.
Cultural control
- Improve drainage of soil before planting so there is no standing water.
- Plant on beds raised 8 to 10 inches to improve drainage, or use drain furrows. In home gardens or small plantings, raise the bed 15 inches.
- Preplant soil solarization has been helpful in reducing populations of certain soilborne pathogens and weeds in western Oregon. Place clear plastic (preferably anti-condensation film) directly on smooth, rototilled ground, which has been irrigated to field capacity and then allowed to drain for 1-2 days. Bury the edges of the plastic to trap the heat. Solarize for 4 to 6 weeks (or longer) during the hottest part of the summer, beginning in early- to mid-July. Use in combination with other techniques.
- Do not plant in draws or swales where surface water flows after heavy rains.
- Chisel or subsoil 18 to 20 inches deep between rows in late fall to increase drainage.
- Use of straw mulch is beneficial to prevent fruit from touching the soil and spores from splashing on fruit.
Chemical control Materials useful for gray mold are not useful for leather rot but initial timings will be similar. Alternate between chemicals with different modes of action to prevent or delay building up resistant microorganisms. Limit the use of any one group during crop production. Ressitance to FRAC 4 and 11 fungicides has been reported in southeast USA. The FRAC 4 and FRAC P7 fungicides used to manage Phytophthora do not kill this organism. They can only prevent establishment of the organism before it gets into the plant. They can also prevent continued growth if the organism is already inside the plant, thereby delaying symptoms that might have developed. Once chemical activity has subsided with time, the organism can resume growth within infected plants.
- Abound at 6 to 15.5 fl oz/A. Do not apply with silicone-based surfactants. May be applied on the day of harvest. Group 11 fungicide. 4-hr reentry.
- Agri-Fos at 1.25 to 2.5 quarts/A. Group P7 fungicide. 4-hr reentry. WA only.
- Aliette WDG at 2.5 to 5 lb/A. Do not use with adjuvants. Can be applied on day of harvest but with a 12-hr preharvest interval. Do not apply within 30 days of harvest in British Columbia. Group P7 fungicide. 12-hr reentry.
- Fosphite at 1 to 3 quarts/A. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- MetaStar 2E at 2 quart/A. Group 4 fungicide. 48-hr reentry.
- OxiPhos at 2.5 to 5 quarts/A as a foliar spray. Group P7 fungicide. 4-hr reentry.
- Phostrol at 2.5 to 5 pints/A. Group P7 fungicide. 4-hr reentry.
- Rampart at 1 to 3 quarts/100 gal water/A. Group P7 fungicide. 4-hr reentry.
- Ridomil Gold SL at 1 pint/A. Group 4 fungicide. 48-hr reentry.
- Topguard EQ at 5 to 8 fl oz/A. Do not use with silicone surfactants. May be applied on the day of harvest. Sprayers should not be used on apples. Group 3 + 11 fungicide. 12-hr reentry.
References Eikemo, H., Klemsdal, S.S., Riisberg, I., Bonants, P., Stensvand, A., and Tronsmo, A.M. 2004. Genetic variation between Phytophthora cactorum isolates differing in their ability to cause crown rot in strawberry. Mycological research 108:317-324.
Marin, M. V. and Peres, N. A., 2021. Improving the toolbox to manage Phytophthora diseases of strawberry: searching for chemical alterna-tives. Plant Health Progress, https://doi.org/10.1094/PHP-02-21-0034-FI.