Warm nights, humid afternoons and a canopy that has closed in. Here are six that are showing up in vegetable beds right now, and what really works against each.
July is when the vegetable garden stops looking like a magazine. The plants are big, the canopy is closed, and somewhere down in that green tangle a leaf has broken. Warm nights and humid afternoons are ideal conditions for fungal and bacterial problems, and once held in a packed bed it moves quickly, usually faster than anyone notices.
Catching it early is the whole game. Host common diseases and pests of vegetable gardens they announce themselves on a handful of lower leaves before the plant is really in danger, and pulling those leaves plus a well-timed spray is often enough to hold the line. Wait until half the plant is yellow and you are no longer dealing with anything, just dealing a loss.
6 vegetable diseases to deal with this month
The six below are what appear in July in plots in many climates. Each has something to say, and each has an answer that works if you gain control soon enough.
1. Early Blight
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Dark spots with concentric rings in them (like a tiny target) that appear on the older, lower leaves of a tomato or potato. These rings mean early blight (Alternaria solani). Around each point the leaf turns yellow and continues until the leaf gives up and falls, and the fungus is already working on the one above it. It overwinters in soil and old plant debris, so rain and water from hoses splashing onto the lower leaves is how it starts each year.
Cut off affected leaves as soon as they appear and do not compost them. The mulch under the plants prevents splashing. If it spreads, that copper fungicide from Home Depot it is approved for organic gardens and can continue until harvest.
2. Septoria leaf spot
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People confuse it with early infection, understandably. Septoria leaf spot (Septoria tomatoes) produces smaller spots, rounder and lots of them — you can count dozens on a leaf, light gray in the middle with a dark rim around the edge. Tomatoes get it the worst.
Timing is the hard part, as it tends to appear just as the fruit is growing, and a plant that has lost its leaves is not very mature. Treat it as you would early blight. Strip off infected foliage, water at ground level rather than above, thin any stems that crowd together to allow air to move. Copper fungicide it will slow down the spread. It will do nothing for a card that is already detected.
3. Measles
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Squash, cucumbersand melons take it almost every summer. What you are looking at is powdery mildew (Podosphaera xanthii), and it really does look like someone poured flour on the leaves — a few dusty patches at first, then those patches grow into each other until the surface is chalky all over.
Unusual among them, it does not need wet leaves to spread. Moist air and shade will do, so it settles in the middle of a dense plant. The leaves eventually turn black and the fruit below ripens poorly. Pull off the worst leaves, open the plant and treat the rest. Bonide’s neem oil from Amazon It works as a fungicide and an acaricide at the same time, which is handy in a bed that has both problems.
4. Downy mildew
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This is a completely different disease, despite the name. Mildew (Pseudoperonospora cubensis) appears as yellow patches on cucumber and melon leaves, oddly angular, trapped by the veins on both sides. Turn over a leaf on a wet morning and underneath is a fuzzy gray or purple growth. Royal downy mildew it will also tear up your herb garden if left untreated.
It moves faster than powdery mildew and, unlike its namesake, needs moisture. Once a plant has it, there’s no real cure, which leaves protection as a unique move. Give plants space, keep water off the foliage, and apply a preventative fungicide to nearby healthy leaves before spores land on them.
5. Bacterial leaf spot
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Peppers and tomatoes they both get bacterial leaf spot (Xanthomonas spp.). It’s the worst problem of the bunch, mostly because the bacteria wash away the fungicides that handle everything else here. Early on, the spots appear small and dark, water-soaked, sometimes with a yellow halo. In fruit they harden into raised crusts.
Splashing water carries it, as do hands and tools working through wet foliage, which is reason enough to stay out of bed until things dry out. Copper fungicide is the only chemical option, and even then it protects healthy tissue rather than repairing what has been damaged. Anything badly infected is better pulled than nursed.
6. Fusarium Wilt
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Watch for a plant that wilts on one side in the warm part of the afternoon, comes back overnight, then does it again the next day. Give it a week and that branch is dead and yellow while the rest of the plant carries on like nothing happened. Fusarium wilt (Fusarium oxysporum) is down in the soil and works from the inside — it blocks the tissue that transports water through the stem. Cut a wilted stem open lengthwise and lo and behold, a brown strip just below the skin.
Nothing cures it. Once this fungus is in a bed, it remains for years, so what remains is selecting resistant varieties and crop rotationalong with keeping plants from the drought stress that seems to invite it in the first place.
