During the last summer many tomato field in Bulgaria were attacked by a new powdery mildew, which has symptoms quite different from those caused by Leveillula taurica. Unlike the sporulation of L. taurica, that of the new fungus is only on the upper side of the leaves. It starts like a single white mildew spots, later enlarge in size and cover almost completely the leaf surface. It is interesting to note that the new powdery mildew can attack also petioles and stems. There are variety differences in susceptibility as far as stem symptoms are concerned. The fungus produces dark colored cleistothecia on the mycelium.
The above mentioned symptoms have been observed in 1986 (Stamova, 1987) but no identification of the causal agent have been done. The investigation carried out during 1989 points out that the pathogen belongs to Erysiphaceae family.
After artificial inoculation of different crops and weeds we recorded white mycelium on R. sativus, P. sativum, Ph. vulgaris, Ph. acutifolius, G. hispida, L. luteus, D. stramonium, C. sativus, C. edulis, and S. melongena. The following crops didn't show any symptoms: all cereals, C. pepo, C. annuum, P. sativum, L. esculentum, D. carota, L. usitatissimum, B. vulgaris, A. hypogaea, S. hortensis, P. anisum, C. carvi, T. pratense, F. pratensis, Al. pratensis, Ph. pratense, D. glomerata, L. perenne, L. multiflorum, B. inermis, L. sativus, M. officinalis, and A. graveolens.
We are still working to identify the causal agent of the new powdery mildew which is already spread in many Euroworkingries (personal contacts with Dutch, French, and Russian research workers and breeders).
Literature cited:
Stamova, L. 1987 Breeding for disease resistance in tomatoes L. esculentum Mill., Dr. of sci. thesis, Plovdiv, Bulgaria.