Watch out for angular leaf spot of strawberries
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EmailIn the Midwest, angular leaf spot (also called bacterial blight) is the only reported strawberry disease caused by a bacterium, namely Xanthomonas fragariae. This bacterium has been hitchhiking around the United States on strawberry planting material, since it was first reported in Minnesota in 1960. Although the disease has not been a major problem, it can occasionally become serious. Economic damage is mainly due to blackening of berry stem caps, which mars the appearance of berries. However, severe leaf spotting can also result in premature leaf drop which may affect plant vigor and yield. Among strawberry cultivars, Allstar, Redchief, Glooscap, Kent, Lester, and Lateglow are known to be fairly susceptible.
Typical symptoms of angular leaf spot are small, angular water-soaked
spots on the lower leaf surface. Spots may coalesce resulting in larger
lesions and necrotic areas. On the upper leaf surface, the lesions look
like irregular reddish-brown spots and could easily be mistaken for
scorch. It is important, therefore, to inspect both the upper and lower
leaf surface. Angular leaf spot lesions are distinctly angular and
translucent when the leaf is held up against the light, whereas scorch
lesions are more rounded and not translucent. Under humid conditions, a
shiny or slimy bacterial exudate can be seen on the lesions on the lower
leaf surface. The exudate eventually dries out into a scaly, whitish
film. Heavily infected leaves may die, especially if major veins are
infected, and the infection may even become systemic. The pathogen can
infect all plant parts, except berries and roots. However, berry stem
cap infections can be serious, resulting in blackened caps and
unattractive fruit.
The bacteria overwinter in old infected leaves and crowns. Primary
infection of new growth in the spring occurs by rain or irrigation water
splash. The bacteria enter plants through wounds or by actively
swimming into natural plant openings (such as stomata, the plant’s
breathing pores) aided by dew, rain or irrigation water. Development of
the disease is favored by moderate to low daytime temperatures (around
68ºF), low nighttime temperatures (near or below freezing), and high
relative humidity. Long periods of leaf wetness due to heavy dew,
irrigation, or prolonged rains also favor disease. Young, vigorous leaf
tissues are more susceptible to the disease than older leaves.
Angular leaf spot can be managed by using clean planting material,
adequate plant and row spacing, and removal of infected plant debris
after harvest. If leaf lesions are common during fruit development and
the weather is conducive, there is a risk of berry stem cap infection.
It is therefore important to protect the berry stem caps from infection
by applying protective sprays. Copper products, such as Kocide and
Cuprofix, applied on a preventive basis are the most effective products
for control, but care has to be taken to avoid phytotoxicity, which
manifests itself by purplish discoloration on leaves. Adding lime as a
safener is recommended, particularly since the cool, slow-drying
conditions that promote the disease also promote copper uptake by the
plant.
