(See photos in the previous post)
In my last post, I reported having seen some potato plants in San Francisco with what looked like early blight, a disease caused by the fungus Alternaria solani. I remembered from my research for the book Controlling Vegetable Pests, which I wrote in the early '90s, that this disease is not common here in California, but much more so elsewhere in the US. I wrote then that it is "less common in arid regions of the West, although overhead irrigation and frequent heavy dew promote disease in these areas." It showed up the week after we had an unusual June rain, and in a garden that has frequent fog. But the question is: Where did the spores come from?
My first thought was that someone had planted potatoes from the grocery store. These could carry the spores, even without symptoms, or could have had dark lesions with underlying brown or blacky corky rot from spores that grew on their surfaces. However, the gardener assured me that these were purchased at a nursery as certified disease-free.
So the spores must have blown in from a nearby garden or were carried in the rain. Then they landed on potato leaves that had some injury, at a time when the temperature and amount of hunidity were right for them to start growing, and the rest is history. The ideal temperature for early blight spore growth is 75-80 degrees F, but the symptoms develop fastest at 70 degrees.
This disease can also infect tomato, and, occasionally, eggplant or pepper. Wild relatives of these plants are also susceptible. On tomato, the leafe symptoms, the bull's eye spots, are the same as on potato, and there may be sunken lesions on older stems. In additon, there may be excessive blossom drop (common here anyway due to cold nights) and leathery, sunken, dark brown lesions on the stem end of the fruits.
If this disease shows up in your garden, take the plants out of the garden. (In San Francisco, they can go in the Green Bin, since the municipal composting system will heat up enough to kill the spores.) If potato plants have formed tubers, eat them up soon, as they may decay in storage. Don't store them where you will kep other, healthy, potatoes later.
The spores can germinate in plant material from infected plants as long as it has not completely decayed, so pick up every bit of the plant and dig up most of its root. Do not save seeds from tomatoes that ripened on infected plants, as the spores can hide in the seeds.
Remove any solanum weeds that are growing in your garden, as they can harbor the disease. (They have flowers similar to those of tomato, but usually with white or pale lavender petals, and have clusters of small, inedible berries that turn black when ripe.)
Plan not to grow tomatoes or potatoes in the place where infected plants grew for two to three years.
When you do plant potatoes or tomatoes, provide your plants with adequate water and fertilizer and avoid getting water on the leaves of tomato'potato family plants. Start examining them when they are about a foot tall to be sure they do not have symptoms. Prevention is more successful than cure when it comes to plant diseases, but if you have had the disease, or if you see the first lesions it is causing, you might try the fungicide Serenade, which is based on a soil bacterium. Follow directions on the label, including repeated sprayings as it directs. (If you are spraying after seeing lesions on a leaf or two, pick off those first infected leaves before you spray.)
May early blight of tomato or potato not darken your door, or the leaves of any plants you are growing to produce food!