2.5.28
EFFECTS OF PHYSICAL AND CHEMICAL FACTORS OF SOIL ON THE SPOROLATION AND GERMINATION OF CYSTS OF THREE SPECIES OF PHYTOPHTHORA

AR RABIEIFAR1, A ALAVI1 and A ALIZADEH2

1Plant Pests and Diseases Research Institute, Evin Tehran, Iran; 2Department of Plant Pathology, Agriculture College, Tarbiat Modares University, Tehran, Iran

Background and objectives
A search in the literature reveals disease in more than a thousand combinations of host plant and species of Phytophthora. Many are of only minor import but some attain the status of major epidemics [1]. It is becoming more apparent that the physical and chemical factors of soil influence the development of all diseases caused by Phytophthora spp. and their geographic distribution [1]. Looking at the distribution of different species of this genus in Iran [2] our objectives were to determine the effects of temperature, pH, cations, anions, sugars and amino asids on the sporulation and germination of their cysts.

Results and conclusions
Maximum sporulation of Phytophthora drechsleri, P. ;citrophthora and P. ;nicotianae occurred at 20, 15 and 25C and at pH 6, 7 and 5, respectively. Among the cations Fe, Mn, Cu and Zn inhibited their sporulation but with the exception of P. ;nicotianae K, Na, Mg and especially Ca enhanced it at up to 100 ;mM. Among the anions, nitrate and sulphate enhanced the sporulation of all the fungi up to 50 ;mM, but the rest (carbonate,nitrite,phosphate,sulphite) were inhibitory, especially at concentrations of 10 ;mM or higher. All of the sugars (glucose, saccharose, sorbitol, galactose) and the amino acids (methionine, arginine, asparagine, glutamic acid) reduced the sporulation of all the fungi compared with that of the control. The amino acids proved to be especially inhibitory at concentrations greater than 1 ;mM. The maximum percentage germination of cysts of P. ;drechsleri and P. ;citrophthora was at 15-20C and 25-30C and at pH 8 and 5, respectively. Ca and Na increased the germination of cysts of both fungi up to 50 ;mM but Fe, Cu, Zn and Mn inhibited it greatly. Among the anions, nitrate and sulphate enhanced the cyst germination of both fungi but the rest were inhibitory, especially at 10 ;mM, or higher, except for cysts of P. ;drechsleri which germinated more than the control at 10 ;mM of sulphite and carbonate. With the exception of glutamic acid, all sugars and amino acids, especially glucose and methionine, increased the percentage cyst germination of both fungi.

References
1. Erwin DC,Bartnicki-Garcia S, Tsao PH, 1983.Phytophthora: its Biology, Taxonomy, Ecology and Pathology. St Paul: APS Press, pp. 392
2. Ershad D, 1992.Phytophthora Species in Iran. Tehran: Agricultural Research Organization, pp.217.