Greens' Diseases & Insects
Lettuce is generally a carefree crop. Occasionally some plants, especially those varieties that form heads, will be hit by a fungus disease called bottom rot.
Bottom Rot Details
Rusty colored spots appear on the lower leaves first, eventually spreading until the entire head is rotted. As soon as you notice any infected plants, harvest them, cut away the still usable portions to eat, and destroy the rest. Clean up the garden well at the end of the season to reduce the number of disease-causing spores that overwinter in the soil, and rotate your crops so that lettuce is in the same spot only once every four years, if possible.
Another common ailment of head lettuce is tip burn, which causes the edges of many leaves to turn brown and die. It's mostly a hot-weather problem and isn't caused by a fungus or insect. Tip burn is usually most severe if there's a lot of fluctuation in soil moisture. Try to keep lettuce plants evenly moist and choose varieties resistant to tip burn, such as 'Ithaca' and 'Simpson Elite'.
Be sure to thin your wide rows of greens properly so the plants have enough air circulation to dry off after a rain or watering. If they're too thick, plants may stay wet too long and develop rot. Continual wetness is an invitation to disease.
Beet and chard greens sometimes develop leaf spot trouble. Spots develop on the leaves, usually most severely on the older leaves. Infected leaves may turn yellow and die. Damage is seldom serious enough to warrant any control other than picking off and destroying infected leaves. If your greens are hit with a hard case, applications of an approved fungicide can help to bring the problem under control.
Remember, it's a good idea to rotate the location of the cabbage family greens (kale, collards and mustard) in the garden each season to help avoid disease problems.
Insect Pests on Greens
There are few insects that generally cause serious problems on greens crops. Probably the most troublesome for many gardeners are the small leaf miners that feed on spinach, chard, beet and turnip greens. The immature stage of a small fly, these tiny larvae tunnel in between the layers of leaf tissue, feeding and causing tan-colored blotches on the leaf surface. A good way to control miners is to cover the rows of greens with screen cages or floating row covers to keep the adult flies from laying their eggs on the leaves. Also, examine leaves closely for clusters of white eggs on the undersides of leaves, then spray with an approved insecticide to kill emerging larvae. Spraying after the miners are inside the leaves does no good.
Aphids and Friends
Aphids and leafhoppers can be a nuisance in some gardens. These insects can spread certain diseases among lettuce plants. Aphids are soft-bodied, pear-shaped and may be green, yellow or purple. Leafhoppers are light greenish-yellow, small but quite active. Spray with insecticidal soap at 10-day intervals to discourage these pests. Keep weeds under control in and around the garden since they can harbor leafhoppers.
The more pungent greens like chicory and curly cress seem to have fewer insect visitors than the milder-flavored ones.
For more information on identifying insects and diseases on your greens, and methods and products to control them, contact your local county Extension agent. He or she will be familiar with the problems that are most likely to be troublesome in your area.