Angular Leaf Spot on Strawberries
Posted: May 31, 2011
It is important to note that because this disease is caused by a bacterium, rather than a fungus, fungicides used to control other diseases will not help have an effect on angular leaf spot.
Symptoms can vary a bit in coloration depending on the plant variety, weather conditions, and time of year. Key diagnostic features, however, are that the lesions are confined by the small veins of the leaf and the infected areas appear as a lighter color when leaf is held up to the light, as the bacteria cause the tissue to clear. Thus, the lesions have a blocky or “angular” appearance, sometimes referred to as a “windowpane” effect. As infected areas accrue, blocks of damage tissue die and turn a brown or red-brown color.
Angular leaf spot also affects the fruit cap (calyx). The caps may have a blackened appearance if they have not had an opportunity to dry out, and after a dry spell may be described as brown rather than black. This is the symptom that often gets people’s attention as the fruit becomes unsalable.
No strawberry varieties have resistance to angular leaf spot. It is systemic within plants, and cannot be eradicated. The bacteria can also invade the plant’s vascular system, causing a general decline, but this is less commonly seen than other symptoms.
Cultural controls consist of minimizing the amount of overhead irrigation used, and any practices that encourage drying of foliage such as keeping plantings weeded. Straw mulch can help minimize water droplet splashing, and avoiding moving equipment or harvesters through wet foliage will minimize the spread of inoculum. As mentioned above, commonly-used fungicides don’t help with controlling this disease. Copper can help though phytotoxicity symptoms, which will appear as a general reddening or purpling of the leaves, may appear if more than four or five sprays are used. Avoid applications when temperatures are warm (higher than the mid-70s). Copper also tends to accumulate in the soil, so routine use without a reason is not recommended. Making applications early in the season will minimize infected leaf material, and prior to wet spells as temperatures become warmer may help to protect the caps. The main purpose of copper sprays is to protect tissue from infected bacterial slime, so if you are in a dry period, you can skip sprays for a while and save them for when you might need them later. Copper hydroxide formulations may be more effective than copper sulfate formulations.