Sergey Elansky
One of the most dangerous diseases of potatoes and tomatoes is late blight caused by oomycete Phytophthora infestans (Mont.) De Bary. This phytopathogen is highly harmful, because under suitable weather conditions, it is capable of massively developing and causing large crop losses, as well as strong variability, which allows it to quickly overcome the resistance of varieties and the toxic effect of fungicidal preparations. To date, no potato and tomato varieties have been created that are completely resistant to these pathogens.
The only option for protection against late blight is the use of plant protection chemicals. Epiphytosis of late blight is initiated by the primary inoculum. In Europe, the primary inoculum is considered to be an infection that got into the soil with diseased seed tubers, oospores (thick-walled reproductive structures P. infestans), as well as zoosporangia brought by the wind from plants grown from overwintered tubers in last year's fields ("volunteer" plants), or on heaps of tubers discarded when storing them for storage. Of these, plants grown on heaps of discarded tubers are considered the most dangerous source of infection. there, the number of sprouted tubers is often significant, and zoosporangia can be carried from them over long distances. The rest of the sources (oospores, "volunteer" plants) are not so dangerous, because it is not customary to grow plants on the same fields more often than once every 3-4 years. Infection from diseased seed tubers is also minimal due to the good seed control system.
In general, the amount of primary inoculum in European populations is limited, and therefore the increase in the epidemic is rather slow and can be successfully controlled using chemical fungicidal preparations.
In Russia, the situation is radically different. Most of the potato and tomato crop is grown in small private gardens; protective measures are either not carried out on them at all, or fungicidal treatments are carried out in an insufficient number and begin after the appearance of late blight on the tops. As a result, private vegetable gardens act as the main source of infection, from which zoosporangia are carried by the wind to commercial plantings. This is confirmed by our direct observations in Moscow, Bryansk, Kostroma, Ryazan regions: damage to plants in private gardens was observed before the start of fungicide treatments of commercial plantings. Subsequently, the epidemic in large fields is restrained by the use of fungicidal preparations, while in private gardens there is a rapid development of late blight.
In the case of improper or "low-budget" treatments of commercial plantings, foci of late blight appear in the fields; in the future they are actively developing, capturing ever larger areas.
Infection in private gardens has a significant impact on epidemics in commercial fields. In all potato-growing regions of Russia, the area occupied by potatoes in private gardens is several times larger than the total area of fields of large producing organizations. In such an environment, private vegetable gardens can be viewed as a global inoculum resource for commercial fields.
Let's try to identify those properties that are characteristic of P. infestans populations in private gardens. Planting of seed and quarantine control of ware potatoes, tomato seeds obtained from dubious foreign producers, long-term cultivation of potatoes and tomatoes in the same areas, improper fungicide treatments or their complete absence lead to severe epiphytoties in the private sector, the result of which is free crossing, hybridization and oospore formation in private gardens. As a result, a very high genotypic diversity is observed, when almost every strain is unique in its genotype (Elansky et al., 2001), and the distribution of genotypes in populations satisfies the Hardy-Weinberg ratio (Amatkhanova et al., 2004), which testifies in favor of free crossing in populations. Oospores are actively formed in the affected plant organs (Smirnov, Elansky 1999). Planting seed potatoes of various genetic origins makes it unlikely that clonal lines specialized to attack a particular variety will emerge. The strains selected in such a case are distinguished by their versatility in relation to the affected varieties; most of them have a close to maximum number of virulence genes (Amatkhanova et al., 2004; Shein et al., 2009). This is very different from the system of "clonal lines" typical for large fields of agricultural enterprises with a properly installed system of protection against late blight. "Clonal lines" (when all strains of late blight pathogen in the field are represented by one or more genotypes) are ubiquitous in countries where potato growing is carried out exclusively by large farms: the USA, the Netherlands, Denmark, etc. (Goodwin et al., 1994, Dyakov, Elansky, 2007, Cooke et al., 2006). At the end of the 20th century, "clonal lines" were widespread in the Asian and Far Eastern parts of Russia (Elansky et al., 2001), which is apparently due to the use of the same varieties of potatoes exclusively for planting. Recently, the situation in these regions has also begun to change towards an increase in the genotypic diversity of populations (S.N. Elansky, unpublished data).
The absence of intensive treatments with fungicidal preparations has another, direct result - there is no accumulation of resistant strains in the gardens. Indeed, our results show that metalaxyl-resistant strains are detected less frequently in private gardens than in commercial plantings (Elansky et al., 2007).
The nearby planting of potatoes and tomatoes in the gardens facilitates the migration of strains between these crops, as a result of which, in the last decade, among the strains isolated from potatoes, the proportion of those carrying the gene for resistance to varieties of cherry tomato T1, previously characteristic only of "tomato" strains, has increased. Strains with the T1 gene in most cases are highly aggressive towards both potatoes and tomatoes.
In recent years, late blight on tomato began to appear in many cases earlier than on potatoes. Tomato seedlings can be infested with oospores in the soil, or oospores present in tomato seeds or adhering to them (Rubin et al., 2001). Since the end of the 20th century, a large number of inexpensive packaged seeds, mainly imported, have appeared in stores, to the use of which most of the small producers have switched. The seeds may contain strains with genotypes typical of the regions of their cultivation. In the future, these genotypes are included in the sexual process in private gardens, which leads to the emergence of completely new genotypes.
Thus, private gardens are a global "melting pot" in which, as a result of the exchange of genetic material, existing genotypes are processed and completely new ones appear. Moreover, their selection takes place in conditions that are very different from those created for potatoes in large farms: the absence of fungicidal press, varietal uniformity of plantings, the predominance of plants affected by various forms of viral and bacterial infection, proximity to tomatoes and wild nightshades, active crossing and oospore formation, the possibility for oospores to induce a relapse of the disease the next year. All this leads to a very high genotypic diversity of backyard populations. Under epiphytotic conditions, late blight spreads very rapidly in vegetable gardens and huge quantities of spores are released, which fly to nearby commercial plantings. However, having got to commercial fields with the correct system of agricultural technology and protection, the spores that have flown in have practically no opportunity to initiate a severe epidemic in the field, which is due to the lack of clonal lines resistant to 10 fungicides and specialized to the cultivated variety.
Another source of primary inoculum may be diseased tubers trapped in commercial seedlings. These tubers were grown, as a rule, in fields with good agricultural technology and intensive chemical protection. The genotypes of the isolates that affected the tubers are adapted to the development of their own variety. These strains are significantly more dangerous for commercial planting in comparison with inoculum originating from private gardens. The results of our studies also support this assumption. Populations isolated from large fields with properly conducted chemical protection and good agricultural technology do not differ in high genotypic diversity. Often these are several clonal lines, characterized by high aggressiveness and a predominance of fungicide-resistant strains.
Strains from commercial seed material can enter populations in vegetable gardens and be involved in the processes going on in them. However, in a vegetable garden, their competitiveness will be much lower than in a commercial field, and soon they will cease to exist in the form of a clonal line, but their genes can be used in the “garden” population.
The infection that develops on "volunteer" plants and on heaps of culled tubers during harvesting is not so relevant for Russia, because In the main potato-growing regions of Russia, deep winter soil freezing is observed, and plants from tubers that have wintered in the soil rarely develop.
Moreover, as our experiments show, the causative agent of late blight usually does not survive at negative temperatures, even on tubers that have retained their viability. In the arid zone, where the cultivation of early potatoes is practiced, late blight is quite rare due to the dry and hot growing season.
Thus, we are currently observing the division of P. infestans populations into “field” and “garden” populations. However, in recent years, processes have been observed leading to the convergence and interpenetration of genotypes from these populations.
Among them, one can note a general increase in the literacy of small producers, the emergence of affordable small packages of seed potatoes, the spread of fungicidal preparations in small packages, and the loss of fear of “chemistry” by the population.
There are situations when, thanks to the vigorous activity of one supplier, entire villages are planted with seed tubers of the same variety and provided with small packages of the same pesticides. It can be assumed that potatoes of the same variety will be found on commercial plantings nearby.
On the other hand, some pesticide trading companies are promoting "budgetary" chemical treatment schemes. In this case, the number of treatments is reduced and the cheapest fungicides are offered, and the emphasis is not on preventing the development of late blight up to mowing the tops, but on a certain delay in epiphytoty in order to increase the yield. Such schemes are economically justified when growing ware potatoes from low-grade seed material, when in principle there is no question of obtaining a high yield. However, in this case, in contrast to the garden populations, the leveled genetic background of the potato will allow the selection of specific physiological races of phytopathogens, which are very dangerous for this variety.
The tendencies towards the convergence of "garden" and "field" methods of potato production seem to us rather dangerous. To prevent their negative consequences both in the household and in the commercial sector, it will be necessary both to control the assortment of seed potatoes and the range of fungicides offered to private owners in small packaging, and to track potato protection schemes and the use of fungicides in the commercial sector.
Due to the weak development of seed production in Russia, large volumes of seed potatoes are imported from abroad. Together with it, new, possibly highly pathogenic and fungicide-resistant strains of pathogens will be imported, incl. late blight.
In Russia, their gene pool will be involved in the natural process of breeding new genotypes, adapted to our conditions and the defeat of varieties cultivated in Russia.
In the areas of the private sector, there is an intensive development of not only late blight, but also Alternaria. Most owners of private plots do not take special measures to protect against Alternaria, mistaking the development of Alternaria for the natural wilting of the tops or the development of late blight. Therefore, with the massive development of Alternaria on susceptible varieties, household plots can serve as a source of inoculum for commercial plantings.
This work was carried out with the partial support of the Russian Science Foundation (project N 14-50-00029).
The article was published in the journal "Potato Protection" (No. 1, 2015)