Features of the formation of microconidia by the fungus Sclerotinia sclerotiorum (Lib.) de Bary

Abstract

The structure of populations of phytopathogenic fungi is largely influenced by the long-term coevolution of micromycetes of the necrotrophic type of nutrition and host plants. The fungus Sclerotinia sclerotiorum (Lib.) de Bary has been attracting the attention of many researchers for over a century (Bolton M.D., Thomma B.P.H., 2006) because it is widespread in many cultures (Kyryk M.M., Pikovskyi M.Y., Azaiki S., 2012) and is dangerous pathogens that cause significant economic losses to crops. In total, it affects hundreds of different plant species (Boland G.J., Hall R., 1994). Populations of S. sclerotiorum are polymorphic in morphological, physiological and biochemical characteristics, genes of vegetative compatibility, virulence, aggressiveness and other characteristics. In the biological cycle of the development fungus reproduces itself sexually and asexually, producing various structures (sclerotia, mycelium, apotheosis with ascospores, and microconidia). At the same time, the peculiarities of conidial stage formation (microconidia) remain poorly understood.

The presence of microconidia is characteristic of many ascomycetes. At the same time, the analysis of the scientific literature shows that the conditions for the formation of microconidia by the fungus S. sclerotiorum remain poorly understood and their significance is debatable (Devasahayam H. L., Henry L. D. C., 2009; Vinod Kumar S., 2015).

The purpose of the study is to determine the peculiarities of the formation of S. sclerotiorum microconidia and to study their morphology.

The objects of the study were affected by the stem form of white mold samples of plants of soybean, rapeseed, sunflower, peas and dahlia, selected in different soil and climatic conditions of Ukraine. The studies were conducted in the problematic research laboratory "Mycology and Phytopathology" of the National University of Life and Environmental Sciences of Ukraine.

Results of the study and their discussion. The phenomenon of the production of microconidial sporulation by the fungus S. sclerotiorum was first detected in variants of mycelial incompatibility of isolates. Under the above conditions, the formation of microconidia began on the tenth day of incubation of the cultures. The pattern of formation of this sporulation is revealed only for the cultivation of isolates that were incompatible. This type of asexual spores was formed in the zone of interaction of the mycelium incompatible isolates, regardless of the host-plant from which they are extracted.

Macroscopic examination of the zones of formation of microconidia revealed them in the form of single point-shaped, or clustered, light, mucous formations that were among the vegetative hyphae of the mycelium of the fungus. Microconidia were formed in two ways: on long (100-250 μm), single, septed conidiophores, or short conidiophores, groups located on the hyphae (8-12 microns thick). In both cases, conidiogenous cells were also phialid bottles formed, 6-8 microns thick and 15-25 microns in length, with chains on the tops of which were microconidia. The microconidia had a spherical shape, 4-5 microns in size, with a distinct sheath, with a splash in the center, hyaline.

In the scientific literature there are informations about different ways of forming microconidia. Thus, in the studies of Kohn L. M. (1979), microconidia were formed on air mycelium, on the surface of the sclerotia, and on the hymenial surface of the apothecia. The possibility of endogenous and exogenous microconidia formation in short hyphae is also indicated (Devasahayam H. L., Henry L. D. C., 2009). A conidial sporulation of S. sclerotiorum was revealed by N.N. Kirik, I.A. Elanskaya and V.V. Borodai (2000) during incubation of inoculated crushed sclerotia discs of carrot roots at 20-25 °C.

Vinod Kumar S. et al. (2015), while studying the biology and infectious cycle of S. sclerotiorum, which causes mold of carnation stems in India, in addition to ascospores, also observed sperm or microconidia that were individually attached to violets. It also indicates the ability of only certain strains of the fungus to form microconidia in vitro on selective media (Sleight Belinda E., 2001).

According to our research, in extracts of different concentrations from the juice of tubers of Solanum tuberosum L., leaves of Brassica napus L., Glycine max L., Dahlia Cav., as well as in distilled water, glucose solutions and sucrose (1,0; 3,0 and 5,0 %) no microconidial germination was detected. The data obtained are consistent with the literature on the complexity of germination of microconidia [Kohn, 1979] and the formation over a long period of underdeveloped germ tubes [Sleight Belinda E., 2001].

At the same time, the role of microconidia in the biological cycle of fungal development remains poorly understood and debated. In particular, their spermatozoa function has not been established (Kosasih B. D. and Willetts H. J., 1975b; Vinod Kumar S., 2015), their role in vegetative reproduction (Devasahayam H. L., Henry L. D. C., 2009).

The pattern of formation of this sporulation is revealed only in the cultivation of incompatible isolates. This type of asexual spores was formed in the area of interaction of the mycelium incompatible isolates, regardless of the plant-animal from which they are removed.

The study of the biological features of S. sclerotiorum sporulation and its role in the biological and infectious cycles of the pathogen are poorly understood and need further investigation.