Your best line of defense against pests and disease in your garden is prevention. Row covers at transplant or sowing can go a long way. You will not need to remove row covers until flowers appear, allowing plants to get a good, healthy head start. Scout unprotected plants for pests and disease weekly to catch any symptoms early. Attract native parasitic flies and wasps with small-flowered pollen and nectar plants like dill, cilantro, or alyssum. But sometimes pests and disease still plague our gardens! Here's the common ones to watch out for in squash plants.
Squash Vine Borers
Squash vine borer moths hatch out of their overwintering cocoons in the soil. These red bodied, wasp-like moths lay reddish-brown eggs around the stem close to the soil line. Deter moths by covering the base of the stems near the ground with soil, mulch, foil, or fabric material. The first sign of squash borers are eggs laid on the stem near the soil line, next to a pin-head sized hole, and sawdust-looking frass near the hole the borer made entering the stem; then part or all of the plant wilts. You can inject the stem with Bacillus thuringiensis (Bt) to kill the larvae, or use a sanitized knife to cut above the entry hole, remove the larvae, and bury the damaged stem in hopes it will create new roots and recover. Because the caterpillars overwinter and rise again in the same fields, removing and destroying (not composting) infected plants is paramount in preventing ongoing borer infestations.
Squash Bugs attack all squash. Both adults and nymphs suck plant juices, causing damage to the plant, which can result in a reduced harvest. Squash bugs' shiny bronze-colored eggs are most often found on the underside of the leaves, as a group of 10-20, in evenly spaced patterns. Squishing eggs is the most efficient way to eliminate infestation. Eggs hatch in about a week, making regular scouting and squishing necessary. Nymphs are pale green when young, becoming darker as they mature. Adults are fast crawling brownish black, flat-backed bugs. Protect young plants with floating row covers (remove covers prior to flowering to allow pollination, or hand-pollinate flowers.) Handpick all stages of squash bugs from the plant base and undersides of leaves and squish, or put in a bucket of soapy water. Place boards on the ground around plants, and destroy adults found underneath every morning by scraping them into a pail of soapy water. Neem (an extract of the Indian Azadirachta indica tree) has an odor that deters gray or brown adult squash bugs and if they eat it, it will disrupt their ability to reproduce (don't expect an immediate knock-down). Insecticide soap and horticultural oil is effective with contact to young nymphs (adults are very difficult to kill) and will need to be reapplied to survivors or subsequent hatches. Kaolin clay products work to coat the leaf surface with a layer of clay creating a physical barrier for eating and egg-laying. Kaolin clay washes off with rain and will need to be reapplied. Also, sowing successions monthly, using row cover allows you to trap bugs and destroy infested plants starting fresh monthly.
Cucumber beetles looks like ladybugs, but are yellow with black dots or yellow with black stripes. Both beetles spread disease like bacterial wilt and should be controlled as soon as possible. Kaolin clay products create a layer of clay that makes feeding on the plants difficult for the cucumber beetles. Spinosad is a soilborne bacterium that is the active ingredient in other organic cucumber beetle controls. Straw mulch makes beetle movement more difficult and creates habitat for wolf spiders, which prey on cucumber beetles. Intercropping broccoli, nasturtium, or radish with squash has been shown to reduce damage from cucumber beetles.
Powdery mildew, one of the more common fungal diseases in the garden, will first appear as powdery white or gray spots which rapidly spread to cover leaves, tender stem tissues and flower buds. Infected leaves often shrivel and dry, which exposes fruit to sunburn, and may cause small fruit size and poor texture. Powdery mildew thrives in humid conditions, especially with cool nights. Healthy plants are less susceptible to infection by powdery mildew fungi, so keep plants properly fertilized and watered, and thin plants to improve air circulation around leaves and stems. Avoid overhead watering of foliage, and water early in the day so any wet leaves will be quickly dried by the sun. Thorough fall clean-up of plant debris will remove overwintering sites for fungi. Do not compost infected plant material. Organic controls include copper, sulfur, compost tea, horticultural oil plus baking soda, milk, and neem based products. Try a spray of compost tea or baking soda plus horticultural oil, spraying plants thoroughly and frequently every 7 to 10 days to prevent the spread of the disease. Compost Tea, long used as a fertilizer, may also help fight fungal diseases. Begin with a burlap or cheesecloth bag containing 1 gallon of well-aged, manure-based compost. Place in a 5-gallon bucket of water, stir well, and steep it in a warm place for 3 days. Then remove the bag, put the liquid in a sprayer or watering can, and spray or sprinkle the entire plant. Oil Spray combined with baking soda has been found to prevent powdery mildew, also. Mix 1 tbsp. of baking soda and 2 tsp. of horticultural oil in 1 gallon of water, and spray plant thoroughly once a week. Be sure to follow the application directions on the oil label; if the plant you are treating is not listed on the label, test the spray on a few leaves first, waiting 48 hours to make sure there is no spotting or discoloration. Some studies have shown that milk can be as effective as commercial fungicides, suggesting that a 10% solution is effective if used several times a week or 20-50% was needed if used weekly. Dilute milk to the desired percent in water, and add a couple drops of natural soap to help the spray stick to leaves, rather than rolling off.