Angular leafspot is a bacterial disease that affects strawberries, cucumbers, melons, squash, and spinach.
There are several different angular leafspot (ALS) diseases in California, depending on the host plant and the pathogen that causes it:
There is also an ALS variety that attacks common and snap beans, but it has not been seen in California, as far as I know. The disease can also affect cotton, but I don’t figure any of you are growing cotton - though it might be interesting…
Angular leafspot symptoms
At first, the disease appears as tiny lesions on the underside of leaves. As the disease progresses, pale, angular spots appear on the upper surface and grow larger. [These areas are not angular because the bacteria are OCD. Instead, it is because they tend to occur between leaf veins, which dictate the angles.] Eventually, infected areas turn reddish-brown, with a yellow or black halo. In cucurbits, the infected areas look more gray or tan than brown, and in spinach the infected areas are very dark. Lesions often appear next to leaf veins and in the calyx (the structure that surrounds and protects flower buds). The bacteria produce an ooze that looks like mucus in the morning and dries to a scaly, white sheen, as the day progresses. This is probably the easiest way to diagnose angular leafspot. Extreme infections can be mistaken for crown rot.
How angular leafspot is spread
The bacteria that cause angular leafspot overwinter on plant material and in the soil, waiting for one thing, and one thing only: moving water. A raindrop, a sprinkler spray, a squirt from the hose can send millions of bacteria in every direction. They can also catch a ride on garden tools, your shoes, your friends’ shoes, and your pet’s feet. Seeds can also be infected.
Controlling angular leafspot
Using only resistant, certified disease-free plants and seeds is the easiest way to avoid infection. This is one of many arguments against starting plants from grocery store purchases. As tempting as it may be, these plants can harbor many plant pathogens that, once introduced, are difficult to eliminate. Chemicals have not been shown to be effective against angular leaf spot. Crop rotation can reduce the likelihood of this disease getting a foothold in your garden or landscape. You can also help reduce infection by only harvesting when plants are dry. Copper has been shown helpful if it is applied just prior to cool, rainy weather.
Finally, for the sake of your plants' health, avoid overhead watering.
You can grow a surprising amount of food in your own yard. Ask me how!
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