Diversity of Rhynchosporium secalis ( Oud . ) J . J . Davis strains in morphological and cultural peculiarities

Summa r y Biological peculiarities of the rye scald fungus Rhynchosporium secalis (Oud.) J. J. Davis, in one population of North-West region were examined. Seventy-eight isolates, the causal agent of scald, were taken from infected rye plants. This isolates were analalysed on rate of growth on artificial test medium, structure and color and temperature dependence. Single-spore strains were obtained from each natural isolate. Color and structure of some single-spore isolates remained stable through repeated transfers to fresh PDA medium.


INTRODUCTION
Leaf scald, caused by the fungus Rhynchosporium secalis (Oud.)J. J. Davis, is a serious leaf disease of barley (Hordeum vulgare L.) and rye (Secale cereale L.).The disease usually caused 30%-40% yield loss of crop in cool and moist climate areas of Russia.
The disease symptoms of rye and barley are similar and may easily be recognized and distinguished from other cereals blights (C a l d w e l l , 1937, We r r e s G., H i n d o r f H., 1991).In the early stages of development appear the dark bluish-gray with water soaked lesions.These lesions are usually 1 to 2 cm in length increase several times before evident collapse of the tissue.This collapse appears very rapidly especially on barley.Developing lesions tend to form stripe.The scalded area soon dries and the center turns to light gray, the margin to dark-brown.However, lesions of the rye have narrow light or brown margin, which is not well identify or is absent at all.Except for stripe form the scalds, which can form concentric brown rings.Leaves are destroyed by severe infection and that results in complete defoliation of the host.
The most part of researches is devoted to the R. secalis-barley system.Rhynchosporium secalis, causing scald of rye leaves is investigated much worse.
The aim of our research was reveal morphological and cultural peculiarities of R. secalis and stability of these peculiarities in single-spores fungus isolates.

MATERIALS AND METHODS
Rye leaves infected by R. secalis we collected from shooting stage plants in one field of North-West region of Russia.Leaf pieces with scald lesions were sterilized in 70% ethanol and 1% sodium hypochlorite (1:1) for 30 seconds, rinsed in pure water, transferred to PSA with 1% yeastrel.In total, we obtained 78 isolates one isolate from one scald lesion.
The growth of 20 isolates was studied on each artificial medium.
From every rye isolate we obtained 10 single-spore cultures.These single-spores cultures were transferred for five times on PSA for the testing of stability in display of signs.

RESULTS AND DISCUSSION
It is known that isolation and cultivation of R. secalis on artificial medium is complicated, because of fungus growing slowly.So, the colonies of R. secalis become visible usually only after 14 days growing on PSA with 1% yeast extract.Small, lights, yeast-like, growing upward colonies were formed on media (C o j a , 1998; K o n o v al o v a , 1999).
It has been shown that the medium composition influences the fungus growth (table 1).The result of this study has shown that the best artificial medium for growth of R. secalis, isolated from rye leaves is PSA and PSA with 1% yeastrel.On these medium average diameter of colonies reached 14.44 mm to 14.72 mm correspondingly.After 14 days of cultivation the average growth rate of the fungus was 1,03 mm to 1,05 mm per day correspondingly.On water agar and Czapek agar the rye fungus growth was absent.On carrot medium and oatmeal medium, the slow fungus growth was detected.
The comparison of the influence of different media on growth of rye and barley isolates shows that the growth rate of barley isolates on some media differs from that rate of rye isolates (figure 1).For example, the barley isolates grow more rapidly than rye isolates on Czapek and carrot media.Our data confirm the conclusion of other researchers with respect to slow growth of R. secalis on all tested media.
Single-spores cultures of R. secalis, obtained from initial isolates growing more slowly (figure 2).They reach the same size as initial isolates after 30 days on PSA with 1% yeastrel.According to the rate of growth, after 30 days all single-spores cultures could be divided into 3 groups.After another 30-days we can see the difference of growth rate level and only two groups could be distinguished.
The changes were explained from the one hand by appearance of dark pigment in some isolates and presence of chlamydospore (single or gathered in chains) and on the other hand by presence of microsclerotium, which increased as the isolate became older (K a j i w a r a 1968).
The seven morphotypes of rye isolates detected in our study rather differ from barley isolates, dark-colored isolates as usual have mycelia manner of growth but lightly colored isolates have yeast-like type of growth.
The majority of single-spores rye cultures isolated from each group kept structure and color typical for each group.Only small part of light cultures discolored and became dark.Therefore, the high stability allows supposing that the most of singlespore cultures are genetically homogenous.
Thus, the comparison of morphological and cultural properties of rye and barley Rhynchosporium secalis isolates show the significant resemblance of studied parameters.
Our data evidence both existence of specialized barley or rye pathogen forms and physiological influence of host-plants.