Causes for tracheomycosis Ulmaceae Mirb. and the period of their susceptibility in the Lower Volga region

This article presents material covering the results of a study of the causes and conditions of the occurrence of tracheomycosis (graphiosis, Dutch Elm Disease) caused by the pathogenic fungus Ophiostoma ulmi (Buisman) Nannf., as well as the role of waste products of phytopathogen secretions in the death of living cells infected with the parasite. Ulmus laevis Pall., U. minor Mill. and U. pumila L. were used for research. Collection and analysis of disease-infected material have been conducted in plantings in the Volgograd region. For injection, a mixture of pathogen spores had been prepared (500 thousand spores in 1 ml of distilled water). Then injections have been performed directly into the xylem vessels. Plants two to three years old had been selected for inoculation, with twenty plants in each of the three repetitions used. Infestation results for elm specimens have been recorded after five to ten days. To detect the toxicity of fungal secretions on the plant, fungus filtrate was used for preparing the working solution. Seedlings of U. minor Mill. and U. laevis Pall. were used as experimental specimens. It is established that in both variants with two strains, the leaves of seedlings began to shrivel on the third to fifth day, twist into a tube, and acquire the brown color characteristic of tracheomycosis. On the ninth day of the experiment, the leaves of the affected seedlings finally withered. Withering of elm seedlings is a consequence not only of vessel clogging by fungus products, as it occurs in nature, but also by toxic secretions of the pathogen. To determine the timing of elm susceptibility to tracheomycosis, an experiment involving artificial infection of small-leaved elm, which is not resistant to this disease, was conducted from April to October. Analysis of the results showed that intensive spread of the parasite is observed during active growth of the plant, the formation of spring-summer vessels, and wood growth. The duration of the incubation period of graphiosis depends on the timing of infection, and with increasing ambient temperature, the incubation period of tracheomycosis decreases. It has been observed that the high sensitivity of ilm trees to graphiosis lesions is synchronized with the active feeding of insect vectors.