Angular Leafspot

Angular leafspot is a bacterial disease that affects strawberries, cucumbers, melons, squash, and spinach.
 
There are several different angular leafspot (ALS) diseases, depending on the host plant and the pathogen that causes it:

• cucurbits (Pseudomonas syringae pv. lachrymans)
• spinach (Pseudomonas syringae pv. spiniciae)
• strawberries (Xanthomonas fragariae)

 
There is also an ALS variety that attacks common and snap beans. This disease can also affect cotton.

Angular leaf spot on strawberry leaf (Annemik M.C. Schilder) CC BY-SA 4.0

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. 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. Extreme infections can be mistaken for crown rot.
 
How angular leafspot spreads
 
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, or a squirt from the hose can send millions of bacteria in every direction. They can also catch a ride on garden tools, shoes, and pets' feet. Seeds can also be infected.
 
Controlling angular leafspot
 
Using only certified disease-free and resistant plants and seeds is the easiest way to avoid infection. Angular leafspot is one of many reasons starting plants from grocery store purchases is a bad idea. As tempting as it may be, these plants can harbor many plant pathogens that, once introduced, are difficult to eliminate. 
 
Chemicals are not 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 can be helpful if applied just before cool, rainy weather.
 
Finally, for the sake of your plants' health, avoid overhead watering.