As I mentioned last week, I wanted to wait and talk about both Early and Late blights in a separate post as both of these can have serious consequences if they make it into your yard/operation. Late Blight is such an important disease that the USDA has a website for the mapping of Late Blight in the US (www.usablight.org). Not many plant diseases out there have their own website devoted to them! Looking at the website this morning, the nearest confirmation of Late Blight occurred in Portage County, Wisconsin on potatoes in mid-July and on tomatoes in Milwaukee County, WI on July 28th. So the time is ripe for Late Blight to occur.
Let's talk about both Early and Late Blight shall we.
Early Blight is caused by the fungus of Alternaria solani. It first appears on older leaves with blackish/brownish irregular circles that are encircled with yellow halos. As leaves die off, fruit can now be exposed to sunscald and infection on fruit may also be more leathery. Alternaria overwinters on leaf tissue debris and in the soil. Control solutions can include crop rotation, irrigating in the morning, proper spacing to allow greater air flow, and not working when leaf tissue is wet. Older leaf tissue can be removed which in turn can allow for greater air flow.
Late Blight is caused by Phytophera infestans. It only effects potatoes and tomatoes. Phytophera thrives in wet and cooler conditions like what we have been seeing recently. Control for this disease is a problem because it can be wind-borne meaning that fungal spores can move around a operation to operation or garden to garden. You can have the best management practices adopted but it could be a neighbor in turn that allows for the pathogen to get t your crops.
Symptoms for late blight are dark,brownish lesions on the leaves and stems that you can see below. Early blight has much smaller spots when compared to Late Blight. You might also be able to see white spores on the Late Blight leaves. As I've also mentioned before, sometimes one pathogen will make the plant vulnerable for another pathogen.
Late Blight is a very fast moving disease and once it is on the plants it is very hard to control/prevent. There are certain tomato varieties that are resistant to the disease and if you have had a season of late blight, you might consider using a resistant variety. There are some fungicides available as well if caught early on.
Hopefully today we have cleared up some of the main differences of Late/Early Blight. As mentioned, the Master Gardeners Hotline in your county can assist in diagnosing as can I. While you can still get yield from these plants if they have these symptoms, the more they hang around in your garden the greater the possibility of it affecting other plants. I also highly recommend removing all plant material from the garden and either throwing it away/destroying it somehow. Do not compost it.