Angular leaf spot causing leaf necrosis in strawberries
Symptoms of angular leaf spot and copper phytotoxicity have been noted in some strawberry fields.
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, which mars the appearance of berries. However, severe leaf spotting can result in leaf necrosis and premature leaf drop which may affect plant vigor and yield. Angular leaf spot is promoted by cool wet weather, especially with (close to) freezing temperatures at night. Frequent overhead irrigation provides extended wetness for infection as well as splashing water for effective dispersal of bacteria. Among strawberry cultivars, Allstar, Redchief, Glooscap, Kent, Lester, and Lateglow are known to be susceptible.
Typical symptoms are small, angular, water-soaked spots that are most readily visible on the lower leaf surface. Michigan State University Extension advises growers to 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 fungal leaf spots. Angular leaf spot lesions are distinctly angular and translucent when the leaf is held up against the light, looking like small “windows,” whereas fungal spots are more rounded and not translucent. In dead areas on the leaf, this could also look like small, blackish angular spots that are slightly shiny. Under humid conditions, a slimy bacterial exudate may be seen on the lesions on the lower leaf surface. The exudate eventually dries out into a scaly, yellowish or whitish film. 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 68 degrees Fahrenheit), 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 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 fungicides applied on a preventive basis are the most effective products for control, but care has to be taken to avoid phytotoxicity, which manifests itself as purplish discoloration of leaves. Some of this has been seen on strawberry leaves lately and may be exacerbated by cool, slow-drying conditions that result in copper uptake by the leaf.
Adding hydrated or builder’s lime as a safener (e.g., 1 lb lime/lb copper fungicide) is recommended if not using fixed copper products. Read the label of the copper product you are using to know whether and how much lime to add. Applying copper earlier in the day, which allows sufficient drying time, may help prevent copper phytotoxicity. Some newer products, like Cueva and Magna-Bon, have lower copper content and may also have lower risk of phytotoxicity. Biological control agents like Double Nickel 55 (Bacillus amyloliquefaciens) may also help against angular leaf spot.