Angular leaf spot of strawberry—Xanthomonas fragariae
Angular leaf spot is a bacterial disease that is a problem in nurseries and when strawberries in the field are grown during cool, wet weather or are irrigated with overhead sprinklers. Other leaf spot diseases of strawberry are more common than angular leaf spot.
Identification
Infections first appear as tiny, pale green, water-soaked spots on the lower surface of leaves. When viewed with backlighting the lesions appear translucent (semitransparent). When conditions are wet or humidity is high, the leaf lesions exude a thick, dark brown ooze of bacteria and bacterial secretions on the underside of infected leaves. When the ooze dries it appears as a brown varnishlike or whitish film. The translucent appearance of young lesions and their ooze distinguish angular leaf spot from fungal leaf spots, which are opaque (not semitransparent).
The lesions enlarge to become reddish-brown spots that are apparent on the upper surface of leaves. The lesions are angular because they are limited by leaf veins. The angular spots may be bordered by yellowish tissue. As the disease progresses, the lesions and infected leaves become brown and dry and die. At this stage of development angular leaf spot can resemble several other leaf spot diseases of strawberry.
Life cycle
The bacterium persists and overwinters in dried, infected leaves, leaf tissue buried in the soil, and in the crowns of infected strawberries such as transplants from the nursery. Ooze from the leaf lesions is a secondary source of pathogen inoculum when the bacteria are spread by overhead irrigation and splashing rain. Disease development is favored by overhead irrigation and cool, rainy days with cold nights.
Damage
Bacterial infection spots and sometimes kills strawberry leaves. When spots are abundant fruit yield is reduced. In uncommon instances after water-soaked spots appear at the base of newly emerged leaflets, the entire plant may collapse and die. This severe damage resembles that of Phytophthora root and crown rots (Phytophthora species). When strawberry plants collapse, Phytophthora pathogens are more commonly the cause.
Solutions
Obtain certified disease-free strawberry plants from a reputable, commercial supplier. Avoid planting strawberry in the same soil year after year. Each growing season relocate where strawberries are planted.
Plant strawberries in a sunny, open location and on a shallow berm or mound of raised soil. Avoid crowding plants and control nearby weeds. These practices will increase air circulation around plants and improve drainage, which can reduce the abundance of leaf spot diseases.
Where strawberries are grown in warmer areas of the state, especially if strawberries previously grew in that soil, solarizing soil before planting can reduce the abundance of the angular leaf spot bacteria and certain other strawberry pathogens. First create the raised planting beds then cover them with clear plastic for 30 to 45 days during hot, sunny weather. Before solarization, mix residue of a cruciferous crop, in particular foliage and stems of broccoli or mustards, into the soil to increase its effectiveness.
Use drip or furrow irrigation instead of overhead sprinklers. If irrigating overhead, apply the water during the morning when foliage will dry more rapidly compared with irrigating during the evening or night. Carefully manage irrigation to avoid prolonged soggy soil (waterlogging).
Bactericides for application after planting are generally not effective. Pesticides are not recommended for angular leaf spot of strawberry in home gardens.
Adapted from Improvement of Soil Solarization with Volatile Compounds Generated from Organic Amendments, Integrated Pest Management for Strawberries, and Pest Management Guidelines: Strawberry, University of California Statewide Integrated Pest Management Program (UC IPM). |
Reddish-brown lesions on leaflets that are angular because the bacterial infections are delimited by leaf veins.
Angular leaf spot begins as tiny, pale green, translucent (semitransparent) spots.
Angular leaf spot lesions oozing dark brown bacteria and bacterial exudates.
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