Factors Affecting the Vineyard Populational Diversity of Plasmopara viticola

Abstract

Vitis vinifera is very susceptible to downy mildew (Plasmopara viticola). A number of authors have suggested different genetic populations of this fungus exist in Europe, each showing a different degree of virulence. Work performed to date indicates this diversity to be the result of different factors. In areas where gene flow is greater and recombination more frequent, the diversity of P. viticola appears to be wider. In vineyards isolated by geographic barriers, a race may become dominant and produce clonal epidemics driven by asexual reproduction. The aim of the present work was to identify the conditions that influence the genetic diversity of P. viticola populations in the vineyards of northwestern Spain, where the climatic conditions for the growth of this fungus are very good. Vineyards situated in a closed, narrow valley of the interior, in more open valleys, and on the coast were sampled and the populations of P. viticola detected were differentiated at the molecular level through the examination of microsatellite markers. The populations of P. viticola represented in primary and secondary infections were investigated in the same way. The concentration of airborne sporangia in the vegetative cycle was also examined, as was the virulence of the different P. viticola populations detected. The epidemiological characteristics of the fungus differed depending on the degree of isolation of the vineyard, the airborne spore concentration, and on whether the attack was primary or secondary. Strong isolation was associated with the appearance of dominant fungal races and, therefore, reduced populational diversity.

Fig. 1. Plot locations and terrain type.

Fig. 2. Airborne sporangia capture using sticky glass traps facing N, S, E and W. Two supports (A and B) were placed in each plot (at different altitudes if possible), and the traps they carried at two heights (H1 and H2 m) above the soil.

Fig. 3. Micrographs of airborne sporangia (magnification 40×). Plot D had the highest concentration, and Plot A the lowest.

Fig. 4. PCA performed for each plot separately, taking into account primary (oil spot) and secondary infection (mosaic spot).

Table 1. Plot locations, grape variety, altitude, years of plantation (YP), training systems, density of vines, distance between rows (dr), distance between grapevines in a row (dv), total n^o plants (Np), plot surface area, sampling dates, and sample size (SS)

Table 4. Mean disease incidence (%) under field conditions observed on leaves and clusters

Table 2. Monthly temperature (℃), rainfall (L/㎡) and relative humidity (%) at the plots

Table 3. Airborne sporangia concentrations (mean and standard deviation)

Table 5. Allele diversity and possible fungal genotypes

Table 6. Number of alleles and genotypes, observed heterozygosity, gene diversity and polymorphism information content (PIC) of the seven P. viticola SSR loci

Table 7. Summary of population genetics indices

Table 8. Pairwise population matrix for Nei genetic distances (pi = primary infection; si = secondary infection)