Disease Corner
by Emily Gatch

Curly Top is a viral disease that is common in semi-arid areas of the western United States and is an annual presence at the Seeds of Change Research Farm. The causal agent is a virus transmitted exclusively by the beat leafhopper (Circulifer tenellus) in North America. The leafhopper acquires the virus by feeding on host plants, which can include beets, Swiss chard, spinach, beans, and cucurbits, as well as tomatoes and peppers. Curly Top does not seem to exhibit what is known as secondary spread, defined as the expansion of a disease from plant to plant following initial establishment; rather, its occurrence is the result of a "raining" of migratory leafhoppers passing through an area. Since the virus is restricted to the phloem (sap-conducting vessels) of plants, physical handling of plants will not transmit the disease. There are several strains of the virus that vary in virulence—some plant species are immune to some strains and susceptible to others. The virus is transmitted in a persistent manner, which means that after the leafhopper acquires the virus it remains infective for its lifespan.

Symptoms include leaves that thicken and curl downward, become very brittle, and turn yellowish with purple veins. If plants are infected prior to fruit-set, fruit ripen prematurely and are small and wrinkled. Plants infected in the seedling stage typically die; if infection occurs after fruit-set, the fruit may still develop normally.

At the Seeds of Change farm, we cover our tomatoes with floating row cover until the plants need to be staked. At that point, they have passed the vulnerable seedling stage and are better able to withstand visits from the marauding leafhoppers. Other recommended management practices include controlling weed hosts such as the Russian thistle (Salsola australis), also known as tumbleweed, but reining in this classic western weed that rolls across fields on windy New Mexico days would surely prove to be a quixotic venture.

by Emilly Gatch
Greenhouse and Pathology Coordinator