By David Strickland and Kerik Cox
Cornell AgriTech, Geneva, NY
Disease: Angular Leaf Spot of Strawberry
Causal pathogen: Xanthomonas fragariae
When to watch for it: Year round
First line of defense: Purchase disease-free stock
Summary:
Angular leaf spot of strawberry has been introduced to major production areas by the importing of infected plant material. The bacterial pathogen, X. fragariae, is very resistant to desiccation and can survive adverse conditions for extended periods in or on dead plant matter both in transit and in the field. The pathogen cannot overwinter freely in soil, which implies that one key cultural control is the removal of old strawberry plant matter before new fields are planted.
Young, vigorous plants are more likely to be infected during the spring when temperatures remain around 68°F during the day and when the leaf wetness is prolonged by rain or sprinkler irrigation. Symptoms begin as water-soaked lesions that enlarge but are delimited by veins, creating characteristic angular spots. During moist periods, viscous bacterial ooze may be visible on the undersides of infected leaves, which dries to a white film. Severe infections will result in the death of the leaf.
Preventing the introduction of X. fragariae on transplants has proven to be more effective than chemical control measures. If absolutely necessary, applications of streptomycin sulfate and oxytetracycline have been shown to be effective protectants in the early season. Applications of copper ammonium carbonate and cupric hydroxide have also been effective protectants in 6 day intervals. However, greater than 7 applications risks the development of severe phytotoxic symptoms. For organic production, the use of hydrogen peroxide at its highest labeled rate is suggested if disease pressure is severe.
A: Bacterial lesions observed on the underside of an infected strawberry leaf. In direct (transmitting) light, the lesions are transparent. Unlike in B: where the lesions are a different shade of green in reflected light, which is a characteristic feature for identification. C: the underside of a severely infected strawberry leaf.