DEAR JESSICA: I'm hoping you can determine what's wrong with my zucchini plant. There seems to be a mold growing on the leaves, and the zucchini just shrivel up, turn brown and fall off. I've seen bugs on the underside of the leaves as well. I've put soapy water and Sevin-5 Dust on them twice. Please help ASAP so I can have homegrown zucchini before the season is over. — Helen Scully, Miller Place
DEAR HELEN: I can provide the clues, but you'll have to play detective.
The white blotches on your plant's leaves could be a normal sign of maturity, as zucchini foliage often displays silvery blotches as it ages. Or, it could be powdery mildew. If you can rub off the discoloration, then it's the latter, which is typically due to high humidity. This can make the plant more susceptible to insect infestation. The best way to avoid mildew diseases is to avoid crowding plants and to employ proper watering practices: Direct water to the soil, not the plant, and water in the early morning, which will minimize moisture that can get trapped between leaves, and allow for a full day's worth of sunshine to dry up humidity on foliage.
Although this might sound contradictory, a good way to remove powdery mildew, especially on a smaller plant like your zucchini, is to wash it away with water. Again, do this early in the morning. If you're willing to take the extra step of drying each leaf with a soft cloth, all the better, although this isn't necessary.
• If you can't wipe off the spots, they could be caused by leafhoppers (usually not dangerous to zucchini plants), spider mites (you would be noticing webbing between leaves), whiteflies (you would see them fly away when you disturb the plant; insecticidal soap applications are safe and effective) or thrips (but they feed on spider mites, so eliminating them could cause more harm than good).
It's also possible you're seeing aphids or even squash bugs, but neither of these cause blotchy leaves.
Aphids are tiny, pear-shaped pests that suck out plant juices and can transmit disease to plants. Ladybug beetles, their natural predators, are the control of choice, and are available at some nurseries and online. Insecticidal soap sprays also can be effective, but are best applied on cooler, cloudy days or in the early evening. If the infestation is severe, you might consider removing the plant altogether to prevent the infestation from spreading to other plants.
Squash bugs are larger pests that can be deadly to plants. The damage they cause can interfere with plants' water-intake ability. Pesticides containing permethrin are effective against young squash bugs, but not adults, which overwinter in the soil, leading to a never-ending cycle of infestation. To help avoid this, remove all plant debris at the end of every season and turn the soil over in late fall to disrupt the insects. Then avoid planting zucchini or other cucurbit plants (cucumbers, melons, squash, pumpkins) in the same bed for three years.
• Another common pest of zucchini is the squash vine borer. Tiny eggs, which are deposited along the lower portions of plants, bring forth thick white caterpillars with brown heads that bore into stalks and kill the plant while chewing their way out. They cocoon in the soil, where they survive winter and re-emerge in late spring as orange-and-black moths ready to repeat the cycle. Sometimes there's even a second generation in August. Telltale signs are small puncture holes in the bottom portion of the stalk and stems, and frass, or excrement, which resembles sawdust. Plants flower, then simply die, seemingly without cause. Squash vine borers do not appear to be your problem, but it's good to be on the lookout for them if you are growing zucchini.
Keep a close eye out for their red, flattened-oval eggs, and pick them off as soon as you find them. You'll need to be vigilant and hunt at least once a week to avoid damage.
If you notice activity, slit punctured stems open lengthwise near holes using a razor blade, and pick out the borers. Then mound soil around the base of plants to cover the injured portion of the stem. This will encourage new roots to grow. If all else fails, use a product labeled for use against squash borers, such as all-natural Btk, and be sure to follow directions carefully.
• You say you've used Sevin-5, a pesticide containing 5 percent of the active ingredient carbaryl. The label says it's effective against 65 garden pests, including Japanese beetles, ticks and grubs, but it's important to know that Sevin-5 is not effective against aphids. What's more, because it is toxic to some beneficial garden insects, it may kill some natural predators of aphids and actually make the problem worse. Although approved for use on vegetables, I personally wouldn't advise the use of any chemical pesticide on edibles.
Sometimes plants drop fruit during heat waves, but we can rule that out this summer.
• Because your zucchini are turning brown before dropping, I suspect blossom end rot could be to blame. Look at affected zucchini, and if they are turning brown from their blossom end, then plants are experiencing inconsistent watering, which can result in a calcium deficiency. Incorporating dolomite lime into soil at planting time can go a long way toward preventing this, but a quick-fix for established plants such as yours is the application of a calcium spray, such as Endz-Rot, to foliage. Fruit produced after applying the drench should be symptom-free.
• If fruit has not browned at the blossom end, there could be other causes, such as poor pollination. To turn this around, you can actually hand-pollinate your plants. As you've probably noticed, the plants each produce male and female flowers. The male flowers are attached to the plant with ordinary stems. The females have a swollen base that should develop into zucchini. Early in the morning, when the flowers are open, your goal is to transfer the pollen from the male to the female. You can do this by touching a cotton swab to the male flower's pollen-covered anther and then to the dark part in the center of the female flower, which is called the stigma. Alternately, you can pick off a male flower, strip off its petals and touch each female stigma directly with the male anther.