1) SUMMARY

Curly top disease has caused losses to sugarbeet, tomato, bean, cucurbits, and several other plants since its discovery in the U.S. in 1988.  The association between the disease and the sugarbeet leafhopper, Circulifer tenellus, was made soon after.  The first severe outbreak of curly top occurred in 1899.  One-third of the California acreage was destroyed by curly top in 1925, and several factories shut down due to lack of sugarbeets to process.  Yields in the Sacramento Valley averaged only ten tons per acre during the years of 1924 to 1931 due to curly top infection.  the introduction of resistant varieties in the 1940's helped the industry rebound, though outbreaks continue even today.
 

Curly top is widespread throughout the arid and semiarid western U.S.  In California, curly top epidemics have occurred periodically on the west side of the San Joaquin Valley between Stockton and Bakersfield and in the Sacramento Valley.  A new production area between Tulelake, California, and Klamath Falls, Oregon, has been found to be an area with high potential for curly top.  Idaho, Washington, and Utah have all had outbreaks of curly top in the past.

 Spring harvest growers are particularly susceptible to curly top disease.  The specter of virus yellows disease dictates that these growers plant after the weather warms.  Unfortunately, with warming temperatures, the foothill weeds that harbor the disease-carrying   leafhoppers die.  The leafhoppers then migrate to the valley floor and infect seedling sugarbeets, resulting in yield reductions corresponding to level of infection.

Many weeds are reservoirs for the disease.  There are 44 plant families and more than 300 species of plants that are hosts of curly top.  Filaree, Russian thistle, mustard, plantain, and several others are important in California.  The predominant method of control relies on these foothill weeds.  As the foothill hosts begin to die, the leafhoppers congregate on the remaining weeds.  These concentrated leafhopper pockets can then be efficiently sprayed and eliminated before moving to crops in the valley.

The beet leafhopper is the only important vector of curly top.  Beet leafhoppers have an extensive host range, produce many generations under favorable conditions, and can move great distances to locate new host plants.  Leafhoppers acquire the virus in as little as one to two minutes of feeding time on an infected plant and retain the virus for months.  Transmission can occur in less than four hours after acquiring the virus.

 2) CURRENT PEST MANAGEMENT PRACTICES

Chemical Control:  Curly top management in California relies on an integrated approach that includes a unique, cooperative spray control program supported by several commodities.  Populations of leafhoppers are sprayed with malathion as they congregate on "green" plants remaining in the drying foothills.  It is common for this spray program to reduce populations by 95 percent.

This program has been in existence since 1943 and has been effective most years.  Notable exceptions were severe outbreaks of curly top in 1950 and 1966.  This was the result of extremely high leafhopper populations in those years.

The Curly Top Virus Control Program (CTVCP) is supported by an assessment on sugarbeet, bean, melon, tomato and several other crops.  The CTVCP monitors leafhopper populations and issues bulletins to growers describing the yearly curly top potential.  Growers are advised on cultural practices that will aid in controlling the spread of curly top and damage to crops.  Weed control on ditchbanks and abandoned areas is stressed to keep preferred host weeds off the valley floor.  High populations of leafhoppers are noted, and growers are advised of the need and benefits of systemic insecticides on crop plants.

The use of Thimet® as a treatment for prevention of curly top began in the 1960's.  The insecticide is placed several inches below the seed at planting.  A threefold reduction in curly top infection was common with this treatment.  Spreckels' research over several years has confirmed this as a cost effective means of curly top control.  It is still recommended today, particularly if the CTVCP issues warnings that leafhoppers are likely to invade cropland.  The cost of the chemical and application is cheap, and most growers in curly top prone areas consider the application good insurance against the disease.

 Cultural Controls:  Since the 1940's, resistance has been incorporated into sugarbeet varieties.  This has been very successful.  The introduction of rhizomania into California in 1983 resulted in many new varieties to combat this new threat.  In the rush to get rhizomania resistance, curly top resistance was secondary.  Many of the first rhizomania resistant cultivars were susceptible to curly top.  Rhizosen, one of the first rhizomania resistant varieties, was quickly abandoned by growers because of susceptibility to curly top.  Currently, growers have available varieties tolerant to both diseases.

Curly top virus exists in many strains.  Strains have differential reactions on different host plants.  Most produce symptoms on susceptible sugarbeets, and some produce symptoms on "resistant" sugarbeets.  The existence of curly top strains is of concern to the industry.  A seed variety evaluation committee composed of growers and processor representatives regularly assesses the curly top resistance of varieties to be planted in the state.  Even a highly resistant variety will suffer yield loss if infected in the seedling stage.

Changing land use patterns can have a major effect on the current control program.  The drought years of 1986 through 1992 led to the abandonment of cropland on the west side of the valley floor.  Weed hosts, particularly Russian thistle, invaded this area and proliferated.  In effect, this brought prime overwintering habitat of the leafhopper close to the agricultural areas.  It also spread out the areas where leafhoppers could breed and congregate, thwarting the spray program.  Growers reacted by protecting new sugarbeet plantings with systemic insecticides.

Growers attempt to plant at a time when leafhoppers are absent.  Factories rely on sugarbeets throughout the year.  Plantings in May are needed to supply factory operations the following April and May.  Therefore, many growers are forced to plant in May, a time when the potential for curly top is high.  Other growers can plant in the winter and avoid curly top infection without any other control means.  Many years ago, that was the only control practice growers had available.

Growers' attempts to plant to stand are integrated with curly top and plant populations.  High plant populations must be maintained at the same time seed is space planted.  Because sugarbeet leafhopper is a desert insect, it prefers gappy stands and low plant populations.  Growers planting to stand run the risk of low plant populations (if seedling problems occur), resulting in gappy stands that are attractive to sugarbeet leafhoppers.

Biological Controls:  The CTVCP is investigating the introduction of predators and parasites for control of sugarbeet leafhopper.

 3) REDUCED RISK OPTIONS:

There have been conflicting reports of the effect of Gaucho® (imidacloprid) on curly top control.  The low use rate, 45 grams per acre, would make this an exceptionally good alternative to both malathion and Thimet®.  Gaucho® also has low mammalian toxicity and is applied to the seed, which is then coated with a polymer.  Contact with the chemical (worker exposure) would be minimal.  Positive results with Gaucho® could be applied to other commodities plagues by curly top.

  4) CHALLENGES

One of the obvious threats to the curly top program is the loss of Thimet® and malathion.  Both insecticides are pivotal for continued field and regional scale management.  The immediate challenge, in light of the FQPA, is to demonstrate to growers that Gaucho® is an acceptable replacement for Thimet® in the field.

The challenge for regional control of leafhopper will shift to biological controls.  Predators and parasites must be found that are effective in reducing leafhopper populations below levels that will cause economic loss.  Part of this challenge will be to find the funds to search for new biological controls wherever they may be.

Land use issues are a concern to the industry.  Intermittent use of cropland on the west side of the San Joaquin Valley increases summer hosts of sugarbeet leafhopper (Russian thistle).  Overgrazing of land in this area increases winter host of sugarbeet leafhopper such as plantain and peppergrass.  An increase in either will lead to greater and more widely dispersed populations of sugarbeet leafhopper.

Regulatory issues, beyond FQPA, are impacting the CTVCP.  Environmental Assessment (EA) must be made every five years before the BLM and DOT will allow spraying of leafhopper hotspots on land owned by either agency.  This process becomes more difficult each year.  The presence of the California red-legged frog and elderberry longhorn beetle (speculated) has imposed severe restrictions on spraying many areas.  The U.S. Fish and Wildlife Service has proposed that money used for spraying and for research into parasites for leafhopper control be used for water monitoring and pesticide drift assessment.  If this erosion of a program that has worked for 50 years continues, several commodities will be searching for curly top management options.

 5) INNOVATIVE FEATURES IN REDUCED-RISK PROGRAM

The use of seed treatment with a low dosage of pesticides will reduce pesticide exposure to the environment and workers and other concerns.

 6) BARRIERS TO ADOPTION OF REDUCED-RISK METHODS

The biggest barrier to adoption of any new practice, low risk or not, is whether it works consistently.  The sugarbeet industry is innovative because we have to be.  This industry will not survive a disaster from curly top as was seen in 1950 and 1966.  Any new practice must be inexpensive, easy to use, and consistently effective.
 

 REFERENCES

Bennett, C.W. 1971. The curly top disease of sugarbeet and other plants. USDA-ARS. Monograph No. 7. The American Phytopathological Society.

Duffus, J. E. and Skoyen, I.O. 1977. Relationship of age of plants and resistance to a severe isolate of the beet curly top virus. Phytopathology 67:151-154g.

Piemeisel, R. L. 1932. Weedy abandoned lands and the weed host of the beet leafhopper. U.S. Department of Agriculture Circular 229.

Severin, H. H. P. and E. A. Schwing. 1926. The 1925 outbreak of beet leafhopper (Eutittix tenella Baker) in California. Journal of Economic Entomology 19:478-483.

Whitney, E. D. and J. E. Duffus. 1991. Beet curly top in Compendium of Beet Diseases and Insects. APS Press.