Diseases
Ascochyta imperfecta Peck. - Spring and Summer Black Stem and Leaf Spot of Alfalfa.
Systematic position.
The causal agent of the disease was described under several taxonomic names . Ascochyta imperfecta Peck., Phoma herbarum West. var. medicaginis West. et. Rab., Phoma medicaginis Malbr. et Roum. var. medicaginis. Currently most of researchers are using Latin name P. medicaginis (Mitosporic fungi) (Hawksworth et al, 1995).Synonym
Phoma medicaginis Malbr. et Roum. var. medicaginis; Phoma herbarum West. var. medicaginis West. et. Rab.Biological group.
Hemibiotrophic pathogen.Morphology and biology.
All plant organs are affected. Spots of various shape, color, and size develop on leaves: small dark-brown to nearly black, often with pale-yellow halo; larger dark-brown spots, but with lighter center and yellowish border; the largest light-brown or light-yellow spots with dark-brown border. On stems, petioles, and beans the spots are small and dark-brown, at first with oily border that later gets yellowish color. Brown to black fungal bodies are visible on dead stems and petioles from late fall until spring. Diseased organs wither, seeds become weak, brown, with shrunken coat. Spring and Summer Black Stem and Leaf Spot appear from early spring, developing faster than other alfalfa diseases. Sources of infection are dry plant residues and seeds with mycelium and pycnidia of the pathogen. After maturing, if drop water is present, pycnospores go out as mucilaginous pink mass, infecting near-by plants. The pathogen exists only at conidial stage. Incubation period is very short (2-6 days). Pycnidia are spherical in shape with thick dark or light brown coverings. The pycnidia size varies from 31.8 to 254.4 . in diameter. Pycnospores are colorless, oval or cylindrical, with rounded ends, unicellular (sometime bicellular), 5-15x2.5-5 . in size.Distribution.
Spring and Summer Black Stem and Leaf Spot is the most widespread disease of alfalfa in all regions of the crop cultivation, but it is harmful in regions with sufficient moistening and on alfalfa growing under irrigation conditions. High harmfulness of the disease was noted in Georgia and Uzbekistan.Ecology.
Moderate and low temperatures are sufficient for the Black Stem and Leaf Spot development; optimum for mycelium growth and pycnidia formation is 20-22.C and 17.C respectively. Early spring and autumn of previous year with abundant precipitation favor appearance of the disease on alfalfa. Seeds are affected under rainy weather in period of flowering and bean formation.Economic significance.
The causal agent of Spring and Summer Black Stem and Leaf Spot is highly specialized parasite, it can infect only plants belonging to the genus Medicago. The disease harmfulness consists in destroying stems in spring time (drying of growing young shoots and death of some plants) and reproductive organs in period of flowering and bean formation). Protection measures are seed treatments with fungicides, residues destruction, thorough harrowing of soil, use of phosphate-potash fertilizers, arrangement of new crops far from previous ones, cultivation of resistant varieties.Related references.
Dorozhkin N.A., Pekel.naya Т.S. 1985. Biology of Ascochitosis caused agent on alfalfa in BSSR. Doklady Akademii nauk BSSR 29(11): 1039-1041 (In Russian).Ganzhina T.S. 1979. Ascochitosis and common leaf spot of alfalfa in Byelorussia. Abstracts of repots of science-practical conference "Ways of future improving of plant protection in Byelorussia and Baltic republics. Minsk, 27-28 February, 1976. Part 1. Minsk: 86-87 (In Russian).
Hawksworth D.L., Kirk P.M., Sutton B.C., Pegler D.N. Dictionary of the fungi - CAB International, 1995.- 616 p.
Kabakhidze P.A. 1977. Study of harmfulness of alfalfa ascochitosis in Georgia. Proceedings of abstracts of 8th Transcaucasian council session for coordination of research in plant protection. 14-16 December, 1977. Erevan: 39-40 (In Russian).
Karimov M.A. 1980. Regularities of development of fungus fodder alfalfa diseases in dependence on ecological factors. Collection of scientific works of Tashkent Agricultural Institute. Diseases and pests of agricultural crops and protection measures. V. 89. Tashkent: 52-63 (In Russian).
Kovalenko N.M. 1985.Alfalfa black stem in Stavropol Territory. Mikologiya i fitoptologiya 19(2): 150-153 (In Russian).
Nigmanova S. 1965. Biology of Ascochitosis caused agent on alfalfa in Uzbekistan and protection measures. Abstact of PHD thesis. Tashkent: Nauka (Uzbek SSR). 16 p. (In Russian).
Peresypkin V.F.1989. Agricultural phytopathology. Moscow: Agropromizdat. 480 p. (in Russian).
Salunskaya N.I. 1959. Ascochitosis of alfalfa in the USSR. Collection of scientific works of Ukrainian Research Institute. Diseases of agricultural crops. Kiev: Ukrainian Academy of Agricultural Sciences: 83-97. (In Ukrainian).
Salunskaya N.I. 1976. Ascochytosis of alfalfa plants. Mikologiya i fitoptologiya 10(1): 37-41 (In Russian).
Samersova V.A. 1982. Reaction of different alfalfa varieties to ascochitosis and common leaf spot in Byelorussia. Collection of scientific works of Byelorussian Research Institute of Plant Protection. Plant protection. V.7. Minsk: Urozhai: 127-140 (In Russian).
Strukchinskas M. 1971. Caused agents of fungus alfalfa diseases in Lithuania and some other USSR republics. Materials of 1st conference for sporous plants of Ukraine (September, 1969). Kiev: Naukova dumka: 231-232 (In Russian).