Late blight on tomatoes: how to spot it and what to do

If dark, water-soaked patches took over your tomato leaves in a few days and the stem is marked too, it is probably late blight. Bag the whole plant and put it in the trash, because late blight spreads on the wind and removing leaves rarely stops it.

Slow, ringed brown spots on the lowest leaves are a different problem, covered in brown spots on tomato leaves. A plant that droops while the leaves look fine is a wilting question. This is the fast one.

What late blight looks like

Wisconsin Extension names three clues: how fast the disease moves, lesions on the stem, and lesions on the fruit. On leaves, it says the lesions quickly enlarge to black, water-soaked areas that look almost oily. Washington State describes the first marks as gray-green, water-soaked spots that quickly enlarge into dark blotches.

Wisconsin Extension adds that in cool, wet conditions the disease can cause plant collapse and death in as little as 7 days. A plant that looked fine on Monday and half-black by the weekend is the pattern.

Check the stem. Wisconsin says stem lesions are dark gray to black, can form on younger and older stems, and are not at all common with the other tomato diseases. A clean stem with spotty leaves points back toward early blight or septoria, although early blight can also mark stems later on, as brown spots on tomato leaves explains.

Fruit gives a third clue. Wisconsin describes large, usually sunken but still firm lesions, golden to chocolate brown, with relatively distinct rings. Washington State says the blotches are firm, look corrugated or wrinkled, and usually start on the upper part of the fruit. A pale, dry, one-sided patch is more likely sunscald.

Then wait for damp air. Cornell describes white fuzzy growth, especially under the leaves, once it is moist. Gray spores instead are Botrytis, not late blight.

Why a balcony is not safe

Cornell says the disease likes cool, wet weather. It needs moisture to infect, and infection can happen without rain when relative humidity is 90 percent or more. Washington State adds that it can be more severe under sprinkler irrigation. Cornell says to water the soil, not the foliage, and Washington State suggests growing tomatoes where they are protected from rain, such as under a balcony overhang.

It also moves between gardens. Cornell’s plant pathologist Meg McGrath calls it a community disease: the longer a sick plant stays, the more spores it sends on the wind to other gardens and farm fields. Other tomato-family plants carry it too. Cornell lists hairy and bittersweet nightshade, petunias and tomatillos.

What to do, in order

1. Harvest fruit that is clean. Maine Extension says non-infected fruit is usable even when the plant is diseased. Wisconsin says to use it quickly, since lesions can still appear. Check it every day.
2. Do not eat or preserve fruit with lesions. Wisconsin says such fruit should not be eaten or preserved, and should not be canned.
3. Pull the whole plant. Cornell says it is rarely possible to control late blight just by removing affected tissue, and the longer you wait, the more spores leave your garden.
4. Seal it. Cornell advises sealing the plants in garbage bags and leaving them in the sun for a few days before they go in the trash. Maine says to discard the bag without opening it. Washington State also lists burying deeply, two feet or more.
5. Keep it out of the compost. Maine, Washington State and Wisconsin Extension all say not to compost diseased plants. Cornell’s 2010 advice also allows burying the sealed bags underground or deep in a compost pile once the plants are dead, but a balcony bin is not a deep pile, so the trash is the safer route here.

Fungicides are a weaker option. Cornell says that if you wait until symptoms appear, it might be too late to rescue plants, and that fungicides need a regular preventive spray schedule and thorough coverage. Washington State says copper products offer limited control and their use alone is not recommended. On anything you will eat, the label decides what is allowed; choosing a pest treatment for edible plants covers how to read it.

The pot afterwards

Cornell’s greenhouse work found the pathogen can survive for several weeks in affected tomato fruit and can infect seedlings that grow from it. The seedlings could be a source of infection where the time between crops is less than about three months. Seed taken out of affected fruit did not produce symptoms in the same work.

So tip out every fruit, leaf and root fragment before the pot is used again. How to refresh mix and when to replace it is in reusing potting mix, and rotating families through containers is in crop rotation in containers.

Growing again

Cornell’s Long Island trials found tomatoes bred with the Ph2 and Ph3 genes, including Defiant PhR, Iron Lady, Mountain Merit and Mountain Magic, showed excellent resistance to the dominant strain, with few symptoms on leaves and fruit. The same trials found that Legend and New Yorker were as severely affected as a susceptible comparison, because the genes in those varieties do not act against the current strain. Cornell also says resistance does not mean full immunity. Late blight does not travel on tomato seed, so starting your own plants avoids a bought-in infection, Cornell adds.

Sources

• University of Wisconsin Extension, Late blight alert (PDF)
• Washington State University Hortsense, Tomato: late blight
• University of Maine Cooperative Extension, Late blight gardener alert
• Cornell University, Avoid the late blight blues
• Cornell Long Island Vegetable Health, Late blight of tomato