In the autumn of 1986 a powdery mildew never observed before invaded the Netherlands. In a short time it spread over the country and larger parts of Northern Europe. The growth of the mycelium is clearly visible on the uuper surface of the leaves and on the stem. It grows maily on the epidermis and not on the leaf itself..It is different form Leveillula taurica that grows inside the leaves and is visible at the underside of the leaves. It is also distinct from Erysiphe cichoracearum, E. polyphaga and E. polygoni. The perfect form has not been observed yet. Until this is detected this powdery mildew will be called Oidium lycopersici. All modern cultivars of tomato grown in the Netherlands are susceptible.
For the development of a disease test 2 L. esculentum cultivars and L. peruvianum LA 2334, resistant to Corynebacterium michiganense were inoculated with O. lycopersici. The inoculum was obtained from heavily infected tomato leaves. The effect of plant age, inoculum preparation and application and humidity on the symptom expression was studied. The best results were obtained by spraying with a freshly prepared spore suspension of 10^4^/ml and keeping the plants at 100% relative humidity for 5 days after inoculation. At all plant stages infection was easily assessed.
In order to search for resistance a survey was carried out with wild relatives of tomato, 2 accessions of L. cheesmanii, 5 of L. chilense, 7 of L. glandulosum, 12 of L. hirsutum, 3 of L. hirsutum glabratum, 5 of L. parviflorum, 19 of L. piminellifolium, 4 of L. pennellii and 60 of L. peruvianum were tested for resistance to O. lycopersici. The experiment was set up in 9 complete replicates of 1 plant of each genotype per plot.
A great variation in the level of susceptibility between and within species and within accessions was found. Most accessions did not show resistance to the pathogen. Some accessions segregated for plants with or without symptoms. On several plants mycelium growth developed but disappeared after some time. Also necrosis and chlorosis was observed suggesting hypersensitivity. All plants tested of two accessions of L. hirsutum (G1.1560 and G1.1290) and one of L. parviflorum (IVT 731089) did not show any symptom. These accessions were carried on for genetic analyses.