SPREAD AND CONTROL OF POTATO LEAFROLL VIRUS IN THE SOUSS VALLEY OF MOROCCO

Experiments were undertaken on the spread and control of potato leafroll virus (PLRV) in potatoes, Solanum tuberosum L., grown for seed production in the Souss Valley of Morocco. Two locations were considered: Ait Melloul, 10 km from the Atlantic Ocean; and Ouled Abdellah, 90 km inland. Potatoes were colonized by three aphid species, Myzus persicae (Sulzer), Macrosiphum euphorbiae (Thomas) and Aphis gossypii (Glover). At both locations, M. persicae were usually the most abundant apterae found on the plants. Depending upon the insecticidal spray treatments used, apterae could be effectively suppressed, or conversely outbreaks triggered, but there was only a weak correlation between numbers of apterae in the treatments and final incidence of PLRV. Yellow pan trap and bait plant data implicated M. persicae alatae as the main PLRV vector at both Ait Melloul and Ouled Abdellah. Volunteer potatoes and the weed Datura stramonium L. were shown to be potential sources of PLRV inoculum. At the coastal site of Ait Melloul, M. persicae flight activity began earlier in the autumn and continued longer into the spring than at the inland site of Ouled Abdellah. Production of seed potatoes might be most practical at Ouled Abdellah in the autumn since there the hot, dry summers largely eliminate nonirrigated hosts of PLRV and greatly suppress populations of M. persicae. Eradication of volunteer potatoes and D. stramonium from irrigated summer cropland in the vicinity of seed potato fields could be an important adjunct to PLRV control. Spring increase of seed potatoes at either location would be risky because the cooler temperatures favour M. persicae. Ait Melloul is unsuitable for seed potato production in any season because of the proximity of extensive greenhouse production of tomatoes, Lycopersicum esculentum L., and peppers, Capsicum annum L., which provides year round sources of M. persicae and PLRV.