*3.8. The Rate of Spread of Potato Blight*

The first symptoms of potato blight were usually observed 42–73 days after planting, depending on the study year and 44–81 days depending on the variety. The resistance traits of the cultivars studied determined the date of appearance of *P. infestans* on plants and the

infection of 50% of the area of potato leaf blades and the rate of disease spreading on the aerial parts of plants.

Table 4 shows the rate of potato late blight spreading by groups of earliness. Very early varieties showed the fastest spread of this pathogen, while the slowest spread was observed in medium-late and late varieties. The average time of destruction of 50% of the assimilation area for varieties with increased resistance (5–6◦) was 22 days—ranging from 16 (very early varieties) to 31 days (late varieties). The theoretical term of stopping the yield for late cultivars resistant to *P. infestans* (resistance 6–7◦ on a scale of 9◦) differed significantly from the other assessed cultivars by about 7 days.

The meteorological conditions in the years of the study were the most decisive factor in the time pace of *P. infestans* spread. The fastest spread of this pathogen, in all groups of early varieties, was in 2007, the flood year, and the slowest was in the dry year, 2011 (Table 4).


**Table 4.** Infection coefficients of *Phytophthora infestans* in the time.

The analysis of the simple correlation between the *P. infestans* spreading rate and the potato tuber yield was analyzed. Pearson's simple correlation coefficients indicate a high positive correlation between the rate of spread of the plague in individual early age groups (r = 0.87 to 1.00), which results from the internal intercorrelation (Table 5). The relationship between the rates of potato late blight spreading and the size of the total, commercial yield of medium-early to late varieties and the yield of very early and early varieties harvested 60 and 75 days after planting turned out to be significantly negative, which means a negative impact of the rate of potato late blight spreading on the yield general and commercial, as well as the yield of early varieties harvested both in the first and second harvest dates. The strongest negative relationship between the rate of spread of potato blight and the potato yield was observed for the commercial yield of tubers (r = −0.62 to −0.71). For very early and early cultivars, the rate of the spread of the blight did not have a significant negative effect, which results from the short vegetation period of these cultivars and their "escape" from infection with *P. infestans*. The shortest time of destruction of 50% of the

lichen surface was found in the group of susceptible cultivars (very early and early cultivars with a resistance of 2–4◦ on a 9◦ scale).

**Table 5.** Simple Pearson's correlation coefficients between the rate of spread of potato blight and the yield of tubers.


X1—*P. infestans* spreading rate coefficient of very early varieties; X2—factor of the spread of late blight of early cultivars; X3—coefficient of the spreading rate of mid-early cultivars; X4—coefficient of the spreading rate of medium-late and late cultivars; X5—the average rate of the spread of the plague; Y1—total yield of potato tubers; Y2—marketable yield of tubers; Y3—yield of early potato cultivars 60 days after planting; Y4—yield of early potato cultivars after 75 days from planting; \*—significant at *p* ≤ 0.05, \*\*—significant at *p* ≤ 0.01.

Among the assessed groups of potato cultivars, a large variation in the pace of development of *P. infestans* was observed, from the onset of the disease to the cessation of harvesting. The differences resulting from the level of resistance were reflected in the assessment of the destruction of the assimilation area at the end of the growing season. It was found that the destruction of the aerial part decreased with an increasing degree of resistance on *P. infestans*. The lowest values of the destruction of the assimilation area were found in the group of medium-late and late cultivars.
