Tuesday, June 23, 2009
Curly Top and the Beet Leafhopper
Because of the seriousness of the curly top blight in local gardens (actually throughout the Western United States), I have researched the subject and provide the following information.
This top picture is of a healthy tomato plant from my garden.
The second picture is of a Curly Top infected tomato from my garden
Causal Agent: Beet Curly Top Virus (BCTV) Hosts: Tomatoes, beans, pepper, spinach, beets, and cucumbers.
Symptoms: Leaves of infested plants are dwarfed, crinkled, rolled inward, and cupped upward. Veins on the underside of leaves usually have a purple discoloration, may be roughened, and often produce swellings or spine-like outgrowths. Roots are stunted and may exhibit a proliferation of secondary rootlets.
BCTV is transmitted to/from plant to plant by the beet leafhopper, Circulifer tenellus. Both the virus and the beet leafhopper have very wide host ranges. Once acquired by the leafhopper, BCTV is carried for the rest of the leafhopper's life, and thus long distance spread is common. Infected plants are usually scattered in a field. The beet leafhopper acquires the virus from infected crop plants or weeds such as wild mustard and Russian thistle. Only brief feeding periods (minutes) are required for the leafhopper to acquire the virus and transmit it to new plants. Plants begin to show symptoms about 7 to 14 days after they are first infected by a leafhopper. Tomato is not a preferred host for the beet leafhopper; however the leafhoppers transmit the virus to tomatoes while sampling it.
Curly Top facts
a. Curly Top is a blight transmitted by the Beet Leafhopper. Although tomatoes are not its preferred host, leafhoppers are fairly indiscriminate, and tomatoes get infected along with beets, beans, melons, squash, potatoes, spinach, peppers, cucumbers and other garden plants. Even Pumpkins have been infected in some states.
b. When humidity is above 50%, Curly Top is nonexistent; thus the desert southwest is very susceptible to this disease.
c. There is no cure, once infected, the plant fails to thrive, will not set additional fruit, and will usually die. Remaining fruit will be of low quality, underdeveloped, and of poor quality and flavor.
What Doesn’t Work
a. Spraying for the Beet Leafhopper is ineffective since the Leafhopper migrates, usually coming down from hillsides as the weather warms. And although spraying will kill the leafhopper, the damage is done before the leafhopper dies.
b. Commercially, there are some insecticide spraying programs, and soil treatments that are effective, but for the home gardener, they are either not available or impractical.
c. Some of the cultural practices of commercial farmers, are also impractical or irrelevant to the home gardener. For example, planting large, thick fields of tomatoes seems to repel the Leafhopper, but gardeners can’t do this.
d. The State of California is investigating the introduction of predators and parasites for control of Beet Leafhopper, but to date, no solution has been found.
e. Covering tomatoes with a mesh or screen to keep the leafhoppers out. Most mesh is not tight enough to keep the leafhopper out, and if it is tight enough, the mesh will also keep out sunlight and create too much shading, and stunt plant growth.
What Might Work
a. There are possibly two or three resistant varieties, but all tomato varieties are susceptible. Ropac and Columbia seem to be resistant, but no immune, to curly top. Floramerica is another variety reported to be resistant, though it's not been confirmed in our area. Bruce Church in Hurricane experiences about 80% success with Ropac and Columbia.
b. Planting tomatoes later in the season, for the Hurricane Valley this would be mid-May to mid-June. Leafhoppers seem to be less prevalent by this date, and moved on to their more favored plants like Russian Thistle, Mustard, and other weeds.
c. Creating dense stands of tomatoes seems to repel the leafhopper, but for most home gardeners, this is impractical. Still, if you can plant in a square instead of a row, you will probably create some protection against the leafhopper.
d. Eliminating Russian Thistle, plantain, and other weeds is also somewhat effective, since the Leafhopper prefers weeds, particularly the Russian Thistle.
e. Creating an enclosed, clear plastic “greenhouse” over the tomatoes, for the early growing period, in theory, should protect the tomatoes from the Leafhopper, but the “greenhouse” must be tight as to make it impenetrable by the Leafhopper. If this can be maintained until mid-May, when it would probably be impracticable and inadvisable to keep the tomatoes inside the “greenhouse.”
f. Shading, which lowers light intensity and retards evaporation, probably delays leafhopper visits, decreases the infection rate, and reduces symptom expression. However, tomatoes do not like being shaded (they prefer full sun), so there is an adverse affect to this practice.
g. New Mexico State University has tested the use of applying a white kaolin mineral product (3% kaolin suspension) on tomatoes and peppers, and the treatment has proven effective against curly top. But row irrigation or drip irrigation must be used since sprinklers and rain will wash the kaolin off the plant and eliminate its effectiveness. Kaolin is a soft, earthy, usually white mineral...and don't ask me where you get Kaolin. It is probably available someplace.
h. Bruce Church in Hurricane says a spray of reconstituted dry milk is effective (From Utah State info), and he uses it. Dr. Sylvan Wittwer confirmed to me that milk is effective as both a repellent of the leafhopper and as an infection preventative. I tried this only one year and it was effective.