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GREY STEM SPOT IS A NEW DISEASE OF SUNFLOWER IN RUSSIA

VI YAKUTKIN

All-Russian Plant Protection Institute (VIZR), 189620 St Petersburg, Pushkin-6, Podbelsky 3, Russia

Background and objectives
Grey stem spot of sunflower, caused by Phomopsis, has spread widely in Russia in the past 8 years. This disease has become as harmful as white and grey moulds and downy mildew, which cause severe losses in sunflower yield in Russia.

Materials and methods
To assess the extent of the disease, fields were surveyed in different climatic zones of Russia using sampling methods during 1989-97 [1]. We also used the data on registration of disease from the State Plant Protection Station. Laboratory and field experiments were carried out at the All Russian Plant Protection Institute and at Belgorod Experimental Station of the All-Russian Research Institute of Oil Crops, using standard methods. The taxonomic position of the fungus causing grey stem spot of sunflower was determined according to known criteria [2].

Results and conclusions
In Russia Phomopsis was first registered on sunflower in the Stavropol region in 1990 on 5 ha [1]. At present the disease affects more than 2 million ha of sunflower crops in Russia (50% of the total cultivated area), with the percentage plants damaged ranging from 0.01 to 100%. We found that the disease is caused by the fungus Diaporthe

(

Phomopsis) helianthi, previously identified on sunflower in Yugoslavia. The fungus forms only beta conidia in its pycnidia. The perithecia with ascospores appear on infected residues after overwintering. In different climatic zones intensive emission of ascospores, as a rule, takes place in May-July. We have estimated that the temperature optimum for the pathogen in pure culture is about 25C. It was proved in laboratory and field experiments that in Russia the primary source of sunflower grey stem spot infection is infected seeds. Sowing infected material spreads disease to damaging levels, reaching 15% and sometimes up to 40%. Ascospores, a second source of infection, promote the wider spread of the disease. Phomopsis affects sunflower more strongly in zones of Russia where growing period can have short rains and the average daily air temperature exceeds 20C for 3 months. The biological damage threshold of the disease is 5% of totally affected plants. In Russia, with a wide variety of climatic conditions and crop agroecosystems, the Northern Caucasus, Central Chernozem Region, Middle Povol'zhe and South Ural are zones of the greatest risk for grey stem spot of sunflower.

References
1. Yakutkin VI, 1991. The identification of Phomopsis causal agent on sunflower and methods of registration. VIZR, Leningrad, p. 23.
2. Cvetkovic MM, Mihaijcevic M, Petrov M, 1981. Nova Hedvigia, 34.