PRACTICAL RESEARCH
Alexander Kuznetsov, Executive Director, SPSK Ustyuzhensky Potato
Alexander Khutti, Ph.D. (Biol.), Senior Researcher, Mushroom, Viral, Mycoplasma and Nematode Diseases of Potato and Vegetable Crops Federal State Budgetary Institution All-Russian Research Institute for Plant Protection.
One of the most harmful diseases of potatoes is Rhizoctonia disease, or "black scab" caused by the fungus Rhizoctonia solani JG Kuhn in an imperfect mycelial stage. The main danger of rhizoctonia is associated with its biological characteristics and life cycle, allowing the fungus to wage a successful struggle for existence, as well as with increased adaptive abilities, the presence of unique anastomotic groups resistant to some of the modern fungicides, a wide range of affected host plants and infectious structures that preserve its viability in soil for several years.
Part I.
VEGETATION EXPERIENCE
The evolutionary component of the disease is such that rhizoctonia as a pathogen is ideally suited to all stages of the ontogenesis of the host plant and is able to begin its development with the onset of potato development. Various infectious structures of rhizoctoniasis, both in the soil and on the surface of the tuber (in most cases) or in the potato itself (latent form), germinate and infect the plant simultaneously with the appearance of sprouts, under the influence of diffuses secreted into the soil.
The fungus affects all the underground organs of potatoes: roots, stolons, eyes, sprouts, root neck, parts of the stems located in the soil, tubers, as well as the aerial part of the stems at the surface of the soil. The symptomatology is extensive: ulcers on all underground and aboveground organs of various shapes and colors (from light to dark brown, sometimes black), both wet and dry; colorless mycelium on the surface of the tuber or its sclerotic compaction, which is an interweaving of the fungal mycelium from dark brown to black, as well as various kinds of cracks, mesh necrosis.
On the aerial part of the stems, closer to their base, a white mycelium “white leg” is formed - the sexual stage of the fungus, leading to the formation of spores that can infect young tubers. Due to the defeat of the vascular system, the aerial part of the plant also suffers: there is a loss of turgor and wilting, the curling of leaves with a "boat" with the acquisition of a violet-red color, the formation of air tubers, indicating a passing pathogenesis in the underground part of the plant.
Over the past decade, there has been a clear tendency towards the formation of a "white leg" not in the second half of the growing season, but during the period of full sprouting of potatoes. The same trend is observed for the ulcerative form of tubers. The manifestation of this symptom of the disease is massive and often displaces another well-known symptom - black sclerotia. The danger of the ulcerative form lies in its increased harmfulness, a significant deterioration in the quality of potatoes, the difficulty of diagnosis and its absence in the new GOST 33996-2016 “Seed potatoes. Technical conditions and methods for determining quality ”, which came into force on January 01, 2018, which partly contributes to the uncontrolled spread of the disease.
The harmfulness of the disease primarily affects the sowing qualities: the affected tubers can partially or completely lose their germination, leading to a significant loss of plants in the field, fewer stems, a decrease in the keeping quality of the new crop, and losses from the causative agent of rhizoctonia can vary from 10-20 up to 40%.
Thus, rhizoctoniosis (based on a combination of harmfulness factors, increased aggressiveness and virulence, phenomenal adaptability) was selected as a test object in the study on the effect of protectants on it, listed in the “List of Pesticides and Agrochemicals Permitted for Use for 2018” and presented at Russian market.
The list of tasks of the technological experiment included not only the analysis of the effect of dressing agents on rhizoctoniasis, but also tracking in the long term (at least three years of studying the pathosystem: the causative agent of the disease is a plant-dressing agent) of the effect of drugs on the immunity of potatoes, yield: the quantity and quality of the seed material produced, assessment of its keeping quality during autumn-winter storage, as well as determination of the prevalence and harmfulness of other concomitant diseases, etc.
The vegetation experience was established in the production conditions of the Burov LLC farm (Vologda Oblast, Ustyuzhensky District). The climate of the region is moderately continental, the total annual precipitation is 500 mm, sandy loam soils prevail. During the growing season from May to September 2018, the average air temperature ranged from 13,3 to 18,5 ° С; humidity was fixed at a level of 64 to 83%; precipitation ranged from 25,9 to 103,7 mm. Taking into account these data, the territory of the economy was recognized as an ideal test site for the planned technological experiment, since Rhizoctonia prefers moderate temperatures and humidity.
During the experiment, a Labadia potato variety of the elite category was used. The variety was selected as a reference for the combination of several factors: medium early, with a peel of light yellow color (more vivid visualization of the symptomatology of the disease manifested), table setting, with a stable yield, suitable for all types of soils, susceptible to rhizoctonia.
The Labadia variety was tested for compliance with technical indicators (in particular, such as infection with viral, bacterial, fungal and nematode diseases) was carried out in accordance with the new GOST 33996-2016 “Seed potatoes. Technical conditions and methods for determining quality. " According to the results of the analysis, the material used for planting corresponded to all regulatory and technical indicators and was recognized as free from infection, in particular, rhizoctonia.
In the production technological experiment, 12 schemes were applied using a modern line of disinfectants included in the “List of pesticides and agrochemicals approved for use in 2018” and presented on the Russian market (table 1).
Table 3. Qualitative indicators (marketability): yield, weight and fractionation according to the results of a field experiment on testing drugs against potato rhizoctonia disease - Rhizoctonia solani JG Kuhn
The design of the experiment, the laying of experience, conducting research and subsequent recording of the results were carried out in accordance with the guidelines developed by the WIZR staff "Methodological guidelines for the technological testing of fungicides in agriculture, 2009". The laying of the experimental schemes was carried out randomly, in four repetitions on a natural infectious background. The sampling of the samples taken took place from the marked plots (10m2, 30 plants each), on the same date from each plot, respectively.
Tuber dressing was carried out under production conditions using a Grimme GL 34T potato planter with a row spacing of 75 cm. The tested preparation was poured into the tuber dressing device according to the norm recommended by the manufacturer, after which the tubers were planted in the soil.
The counts covered three main phases of potato development, optimal for diagnosing and studying the dynamics of the pathogenesis of rhizoctoniasis: full germination (plant height from 5 to 15 cm), full flowering and the budding phase, as well as the moment of potato harvesting (calculation of marketable potato qualities: yield, weight and factionalism).
The first results of technological tests of dressing agents showed (Table 2) that in the first count (phase of full germination), the untreated control showed the best results, the sprouting of the shoots of which was less than in all other versions of the tested dressers.
Experimental schemes with dressing agents indicate that in all experimental variants, the seedling affection was uniform and amounted to no more than 10% of the total occupied area.
In the second (flowering phase) and third (budding phase) accounting for the dynamics of the untreated control remained at the same level: the incidence of rhizoctoniosis of potato sprouts and stolons was either less or at the level of protectants.
Such indicators indicate that uterine seed tubers planted in soil and free from rhizoctoniasis infection in accordance with GOST 33996-2016 fully develop without the use of chemical protective agents, without experiencing stress due to the chemical pressure exerted on the immune system of potatoes and thus demonstrate the result is better or comparable to protectants and can cope (“get away") from an infection that was present in the soil.
According to the totality of the data, in the dynamics of the three counts (table 2) and during the harvesting of potatoes (table 3), the scheme under No. 4 stood out from the tested dressers: the final yield and commercial qualities were the best.
The lag of the effective indicators of the defeat (prevalence and development of rhizoctonia in sprouts and stolons) in protectants, in contrast to the untreated control, is understandable and an indisputable fact: the use of chemicals leads to a delay in the growth and development of plants, which in turn reduces their immunity and leads to more severe infection with diseases, in our case, rhizoctonia. It is worth noting that this picture is typical for this experiment using planting material recognized as free from rhizoctonia infection according to GOST 33996-2016. In the presence of infection, in one form or another, in seed potatoes (even not falling under GOST 33996-2016), the result will be diametrically opposite.
Thus, the use of high-quality dressing agents, as an example of option No. 4, is justified and affects primarily such important qualities as productivity and marketability, which are one of the main ones in the production of elite seed potatoes.
FROM THE EDITOR: we draw the attention of readers that this material contains data on the results of the growing experiment in the first year of the experiment. In the next issue of the journal, we will publish the continuation of the story: the article will include observations of received batches of potatoes at different stages of storage in the season 2018/2019, as well as the results of the second year of the study.