Keep an eye on angular leaf spot in strawberries
Angular leaf spot, or bacterial blight, can cause severe economic damage if not managed properly.
Angular leaf spot (also called bacterial blight) is the only reported strawberry disease caused by a bacterium, namely Xanthomonas fragariae. Economic damage is mainly due to blackening of berry stem caps that 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 are small, angular, water-soaked spots that are most readily visible on the lower leaf surface. So, make sure to inspect both leaf surfaces. On the upper leaf surface, the lesions look like rather nondescript reddish-brown spots and could easily be mistaken for scorch. Angular leaf spot lesions are distinctly angular and translucent when the leaf is held up against the light, looking like small “windows,” 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, yellowish or 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. Primary infection of new growth in the spring occurs by rain splash. The bacteria enter plants through wounds or 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 in purplish discoloration of leaves. Adding hydrated or builder’s lime as a safener (e.g., 1 lb lime/lb copper fungicide) is recommended, particularly under cool, slow-drying conditions that promote copper uptake by the plant. Read the label of the copper product you are using to know whether and how much lime to add. Applying copper during periods of warm, dry weather will help avoid phytotoxicity.
Dr. Schilder's work is funded in part by MSU's AgBioResearch.