McGrath says the first step in diagnosis is to examine your plants carefully for symptoms. "Early in the day is the best time for this because, with late blight, most spore production occurs the evening before," she says. "It’s also important to know that late blight can affect all parts of the plant, whereas some of the 'imitators' do not. Characteristic leaf symptoms are very large spots, which look water-soaked at first. They then become brown, often with a border of light green, wilted tissue."
The underside of leaf lesions is the best place to look for late blight’s white, fuzzy growth of spores, McGrath adds. "Large, dark brown lesions develop on stems and petioles, and sometimes the pathogen produces more spores on these," she says. "When leafstalks are affected, the whole leaf can collapse. Affected fruit develop large, brown firm areas.”
This is the tomato problem that’s most commonly confused with late blight. The reason? Like late blight, the pathogen causes large leaf spots and stem lesions, and can affect the fruit. You’ll often see these symptoms initially on dead plant tissue, such as flowers and leaves, which the pathogen typically needs to become established before it can attack living plant tissue.
Affected fruit are soft, not firm as with blight. And the pathogen growth is fuzzier and gray or brownish, not white. The bad news is, like blight, once you see this occurring on your plants, you should remove infected leaves or entire plants and burn them, or bag them and put them out with the garbage.
To prevent disease in the first place, apply compost mulches to boost the number of beneficial microorganisms that can suppress the disease. (It's never too late to mulch, in case you were wondering.)
When plants can’t get enough water to their leaves, this kills off large sections of leaf tissue. In contrast with late blight lesions, symptoms of drought stress always extend from the leaf edge, they lack a defined border of wilted tissue, and there is no fuzzy pathogen growth. Also, you don’t see drought symptoms on stems or fruit.
To prevent drought stress, make sure you water smart during dry spells.
Leaf, stem, and fruit spots are all smaller than with late blight, and often have a characteristic concentric ring pattern or target-shape appearance. Pull infected plants and bag them with the garbage, and apply mulches of compost or clean straw to prevent spores from splashes from the ground onto your other plants.
Septoria Leaf Spot.
Similar to early blight, leaf and stem spots are much smaller than with late blight, and often have a characteristic tan-colored center. The fruit is not affected. To lower your chances of having this disease problem in the future, keep your garden as weed-free as possible (the pathogen can overwinter on certain weeds, including jimsonweed and seed horsenettle, among others. For even more detailed information, check out late blight organic management.