4.9.13
MINIMIZING PHENYLAMIDE RESISTANCE - A SUCCESSFUL STRATEGY OF POTATO BLIGHT CONTROL IN JERSEY

RA COLLIER and SJ LE BOUTILLIER

Department of Agriculture and Fisheries, PO Box 327, Howard Davis Farm, Trinity, Jersey, JE4 8UF

Background and objectives
Over 50% of the Island of Jersey is cultivated, of which some 56% is cropped with one potato cultivar. The variety, International Kidney, selection Jersey Royal, is extremely susceptible to Phytophthora infestans, the cause of potato late blight. In addition, 40% of the protected area (glasshouse and polythene tunnel production) is planted with potatoes. With this intensive production pattern which creates a 'green bridge' for nearly 12 months of the year, there is a need to monitor the effective use of fungicides, especially the systemic phenylamides. The effect of a disease control failure due to the development of resistance, as occurred in several European counties [1] would be catastrophic to the future of the agricultural industry in the Island.

Manufacturers, under the guidance of the Fungicide Resistance Action Committee (FRAC), have formulated anti-resistance strategies for the best use of phenylamide fungicides in order to extend their usefulness. These strategies had to be adapted to the situation in Jersey and methods of monitoring the fitness of the fungal population devised in order to ensure growers have an effective spray programme. The disease is statutorily controlled under the Blight Disease (Jersey) Order, 1982, which allows for a fairly high degree of supervision in the growing of the potato crop.

Results and conclusions
The level of resistance found in Jersey has remained relatively stable over the period of testing. 1987-89 were years of high inoculum pressure due to the prevailing weather conditions, and resistance remained between 29 and 35%. Over the past 5 years the mean resistance has dropped to 25%. There was no variation in resistance within 30 isolates tested from three fields, and isolates sampled from volunteer potato plants yielded more resistant populations than expected. The pattern of use of phenylamide-containing products has varied over the years. The three-way mix products containing cymoxanil, oxadixyl and mancozeb were introduced in 1991 and have taken up to 24% of the market. The two-way mix products have been available in the Island since 1978.

The phenylamide fungicides are used in the early part of the season. The strategy for use follows the guidelines outlined below: (i) not applied to crops grown under permanent protection, i.e. glass or polythene tunnels; (ii) not applied to crops grown for seed or non-export ware; (iii) a maximum of three sprays applied to any single crop, the last of which must be before 30 June; (iv) not applied curatively; (v) crops under polythene must be treated immediately on removal of polythene provided no blight infection is present; (vi) only to be applied on field-grown export Jersey Royals; (vii) spraying interval of 10-14 days recommended. The statutory control of this disease has allowed the development of this comprehensive strategy for the use of phenylamide containing fungicides.

References
1. Davidsee LC, Looijen D,Turkensteen LJ, Van der Wal D, 1981. Netherlands Journal of Plant Pathology 87, 65-68.
2. Sozzi D, Schwinn FG, Gisi U, 1992. EPPO Bulletin 22, 306-309.