Several seasons back, Jeff McCormack, Ph.D., founder of Garden Medicinals and Culinaries and Southern Exposure Seed Exchange, shared a tomato pruning method with me that delays the onset of early blight, and reduces the number of leaves lost in the course of the season.
Extension publications often suggest pruning tomatoes to prevent disease by improving air and light penetration. Jeff's method concentrates pruning at the base of the plant by removing leaves that eventually will be lost to early blight anyway. When the lowest leaves are removed just as the first leaf spots appear, you also remove millions of spores. And, because the bases of pruned plants dry quickly, the spread of the disease is slowed because early blight fungi need damp leaves in order to germinate and grow.
What's Early Blight?
The most common leaf-spot disease of tomato, early blight (Alternaria solani) fungi cause leaf spots to form on tomato leaves. Inside irregularly-shaped dry patches (which often have yellow margins), look for small dark rings. These are the fruiting colonies. The grayish powder inside the dark rings are the spores, which splash or blow onto new leaves to form new spots. When spots become numerous, entire leaves wither to brown.
Commercially-grown tomatoes are often sprayed weekly with fungicides to suppress early blight. Organic growers sometimes use copper fungicides, which are often effective, but frequent use may harm earthworms. A few resistant varieties have been developed, but some failed to perform well in field trials, and others fall short in terms of flavor and texture.
With big indeterminate varieties, prune or nip out all leaves that hang within 1 foot of the ground. If you see numerous lesions on the pruned leaves, you can go higher, to 18 inches. See the before and after photos below.
With stocky indeterminate varieties, trim out most of the leaves that touch the ground, but don't get carried away. If the plants have already set a heavy load of fruit, I also trim off some of the newest blossom clusters to keep the fruit:leaf ratio high. See the before and after photos below.
In addition to pruning, drip irrigation discourages early blight because the plants can be watered without wetting the leaves. Mulch after the plants are pruned to retain soil moisture.
Early blight starts with a few small lesions, but quickly spreads in damp summer weather. Eventually, the lower halves of infected plants wither to brown.
Before: Vigorous indeterminate tomatoes usually begin to show early blight symptoms just as they load up with green fruits.
After: Pruning off low leaves eliminates the damp environment early blight needs to spread, which reduces the number of leaves lost later in the season.
Before: Stocky determinate varieties that grow close to the ground are often hit hard by early blight just as the fruits begin to ripen.
After: When pruning determinates, take only the leaves that touch the ground. You can also prune off a few blossom clusters to help keep the leaf:fruit ratio high, which insures good flavor.