When I saw these symptoms on some potatoes in a local garden, I thought they looked like early blght of potato and tomato, a disease caused by the fungus Alternaria solani. But I thought it unlikely, because this is not a common disease in our region. I tried to return to get good photos of the diseased plants, but when I arrived, the plants had been pulled, so my photos show leaves I selected to show the symptoms.
The most diagnostic symptom is those dark brown lesions on the leaves with concentric rings visible in them. Those are from repeated growth of the fungus and release of spores. Symptoms are similar on tomato. Also susceptible are a number of wild potato/tomato relatives and other members of this plant family.
Eventually, the brown spots coalesce so that the entire leaves become brown and die.
(This is an entirely different disease from Late Blight, which is Phytophthora infestans. Search this blog for more on that disease, with photos of its symptoms.)
Potato/tomato early blight is spread by spores, as are most fungi. I sent a sample of the diseased leaves, through the Cooperative Extension, to Brenna Aegarter, a UC potato expert in Stockton to ask for help identifying the disease. She lifted spores from the leaves with Scotch tape and put them under the microscope. Here is what she saw, followed by her reply:
"There is clearly Ulocladium and Cladosporium (which are common “molds” on senescing/decaying leaves) as well as some Alternaria spores. However, the challenge is that there are two types of Alternarias in the world. One type is a common mold and would often be found on dying leaves. The other type are pathogens of plants and infect tissue that is living (and includes Alternaria solani cause of early blight). The ones that are not pathogens have short “beaks” while the pathogenic ones have long beaks (the narrow section of the spore that extends out). Under the microscope I saw Alternaria spores with beaks, but the beaks were not so long as to entirely convince me that they are spores of Alternaria solani. But I have sent my photo to a colleague to see what he thinks. Stay tuned!"
Later, I heard from her to the effect that her colleague had confirmed that the beaked spores are Alternaria solani. If you look at the slide, you will see the ones with a long point on one end. There are the other, decay bacteria spores also, and I think the large, long objects are probably plant hairs.
I will, in my next post, explain this disease and what to do about it.