5.5.22
METALAXYL INSENSITIVITY FOUND AMONG FIELD ISOLATES OF PHYTOPHTHORA CAPSICI

G PARRA AND JB RISTAINO

Department of Plant Pathology, North Carolina State University, Raleigh, NC 27695, USA

Background and objectives
Phytophthora blight caused by the pathogen Phytophthora capsici has caused significant economic losses in bell pepper and cucurbit fields in the US and the incidence of the disease has increased in recent years. The pathogen can be dispersed in soil, with surface water, and via splash dispersal from the soil to foliar parts of plants. Management of the disease relies on modifications in cultural practices, crop rotation, and judicious use of fungicides. An alarming increase in incidence of disease in fields that were sprayed with Ridomil Gold according to labeled recommendations occurred in 1997. Epidemics of Phytophthora blight occurred in bell pepper concurrently with multiple applications of the fungicide. Most of the growers had used metalaxyl for the first time in 1997 on bell pepper and disease was widespread. Metalaxyl insensitivity has not been reported previously in field isolates of Phytophthora capsici. However, selection for metalaxyl insensitive isolates after mutagenesis in vitro has been reported [1]. Insensitivity to metalaxyl has been reported among other Oomycete pathogens including Phytophthora infestans , Pseudoperonospora cubensis, Peronospora tabacina, Bremia lactucae, and Pythium species [2].

Materials and methods
Infected plants were collected from 13 different field locations. Twelve of the fields were located in North Carolina and the remaining site was located in New Jersey. Infected plants were surface disinfested in bleach and plated on selective media to isolate P. capsici. Colonies of the pathogen were transferred to V-8 juice agar or maintained on cornmeal agar slants. Metalaxyl amended V-8 juice agar was prepared at levels of 0 ppm, 5 ppm, and 100 ppm. Screening for sensitivity was conducted by placing agar plugs containing the pathogen onto two replicate plates of metalaxyl amended media at each concentration. Isolates were categorized as sensitive if growth was less than 40% of the unamended control at 5 ppm. Intermediate isolates exhibited growth greater than 40% of the unamended control at 5 ppm but less than 40% of the unamended control at 100 ppm metalaxyl. Insensitive isolates exhibited growth greater than 40% of the unamended control at lOO ppm metalaxyl.

Results and discussion
In total, 149 isolates were collected and screened for sensitivity. Of these, 44 isolates were classified as sensitive, 15 as intermediate, and 77 or 52 % of the isolates were insensitive. The remaining 13 isolates gave variable results in growth. Both Al and A2 mating types were recovered from the same fields and insensitive isolates occurred among both mating types.
A significant proportion of the isolates obtained from infected plants in fields where the fungicide has been recently used were insensitive to metalaxyi. The relative fitness of sensitive and insensitive isolates on fungicide treated plants will be compared in further experiments. Two years ago, our entire collection of isolates was screened and isolates were sensitive to Ridomil 2E. A dramatic shift in populations of P. capsici to insensitivity to metalaxyl has occurred in North Carolina bell pepper fields in a relative short period of time. Future research is in progress to track insensitive isolates with molecular markers and develop alternative management strategies.

References
1. Bower LA, Coffey MD, 1985. Development of laboratory tolerance to phosphorus acid, and fosetyi-Al, and metalaxyl in Phytophthora capsici. Can. J. Plant Pathol. 7, 1-6.
2. Marton HV, Urech PA, 1988. History of the development of resistance to phenylamide fungicides. pp 59-60, In: Delp CE, [ed.] Fungicide Resistance in North America. APS Press, St. Paul, MN.