Common leaf spot of strawberry—Mycosphaerella fragariae =Ramularia tulasnei
Common leaf spot is an important fungal disease of strawberry. It is common on strawberry during the rainy season and where strawberries are irrigated with overhead sprinklers.
Identification
Common leaf spots first appear as circular, dark to purple lesions on the upper surface of strawberry leaves. These spots enlarge to about 1/8 to 1/4 inch in diameter and the center of the lesions turns brown, gray, tan, or whitish, depending on factors such as the age of the leaf and environmental conditions. Numerous spots may merge and kill the leaf.
Elongated lesions may form on calyxes (the berry caps, or green whorl of leafy material on top of the fruit), fruit trusses (stems of flowers that later become strawberries), petioles, and stolons. Tiny dark fruiting bodies that are persistent fungal structures (sclerotia) develop in the lesions. But each sclerotia is only about 0.005 inch (0.13 mm) in diameter and requires magnification to be seen.
Damage symptoms of common leaf spot resemble those of the strawberry leaf scorch fungus, Diplocarpon earlianum =Marssonina fragariae. However common leaf spots have a sharply defined border while the borders of spots from leaf scorch are indistinct. There are also several other leaf spot diseases of strawberry.
Life cycle
The fungus causing common leaf spot is commonly introduced as tiny, black sclerotia on infected plants from the nursery. Sclerotia from a previously infested crop can persist for months on strawberry debris in the soil and then infect subsequent strawberry crops. Fall and winter rains and sprinkler irrigation cause the sclerotia to germinate and produce spores that are dispersed by splashing water and wind-driven rain.
Damage
The fungal lesions interfere with water transport in the plant and promote infection by secondary fungi that can further damage strawberry plants. Some infected leaves or entire plants may be killed by the fungus. Common leaf spot reduces plant vigor and decreases the fruit yield quantity and quality.
Solutions
Obtain disease-free strawberry plants from a reputable, commercial supplier. Inspect plants carefully before purchasing and planting them. Reject or discard strawberry plants with leaf spots.
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 can reduce the abundance of leaf spot diseases.
Use drip or furrow irrigation instead of overhead sprinklers. If irrigating overhead, apply the water during the morning so foliage will dry more rapidly compared with irrigating during the evening or night. Avoid fertilizing with nitrogen when plants are young because this favors the production of excess young foliage that is especially susceptible to leaf-infecting fungi.
Where common leaf spot has been a problem, inspect plants at least weekly for disease presence. If only a few plants exhibit symptoms, remove and dispose of these when they are young and before the pathogen spreads. After harvest remove strawberry debris and dispose of it away from where strawberries are grown.
To reduce further spread of the disease if it is widespread, spray plants thoroughly with Bacillus subtilis, Bordeaux mixture, a copper fungicide, chlorothalonil, or myclobutanil when the first three or four groups of three leaflets are produced. If wet conditions persist, a second application can be warranted about two weeks later. Switch to a different fungicide for the second application because fungicide resistance is common in some populations of this fungus.
Adapted from Leaf Spot Diseases of Strawberry from the University of Florida, Strawberry (Fragaria spp.)-Common Leaf Spot from the Pacific Northwest Plant Disease Management Handbook, and Integrated Pest Management for Strawberries and Pest Management Guidelines: Strawberry, University of California Statewide Integrated Pest Management Program (UC IPM). |
Circular, tan lesions with purple margins on strawberry plants due to common leaf spot fungus.
Circular, tan lesions with dark borders due to common leaf spot fungus.
Common leaf spot, a fungal disease, on strawberry leaflets.
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