PATHOGENS OF POTATO ( Solanum tuberosum L . ) TUBER ( Phytophtora infestans ) OCCURRING IN TREATMENTS WITH FOLIAR FERTILIZATION

The paper presents the results of a three-year exact plot experiment (2008-2010) established in Bałcyny (NE Poland). Three potato cultivars were grown: medium-early ‘Adam’, medium-late ‘Pasja Pomorska’, and late ‘Ślęza’. The experimental factors were foliar fertilizers applied alone or in combination (Basfoliar 12-4-6, ADOB Mn, Solubor DF) and two levels of soil mineral fertilization (N1P1K1-80 kg N × ha, 80 kg P × ha , 120 K × ha; N2P2K2-120 kg N × ha, 144 kg P × ha, 156 K × ha). The experimental materials comprised potato tubers. The symptoms of soft rot (Pectobacterium carotovorum subsp. carotovorum), late blight (Phytophthora infestans) and dry rot (Fusarium spp.) were evaluated in 5 kg potato samples, and were expressed as the percentage mass of infected tubers. The rates of common scab (Streptomyces scabies) and black scurf (Rhizoctonia solani) infection were estimated on 100 tubers collected randomly after harvest, according to a nine-point scale, and were presented as a percentage infection index. In the laboratory, fungi were isolated on PDA medium from potato tubers immediately after harvest and after five-month storage. The incidence of tuber diseases depended on potato cultivars affected. The severity of tuber diseases varied between treatments with two levels of NPK fertilization and foliar fertilization. The lowest number of Fusarium-infected tubers was obtained from treatments where three foliar fertilizers were applied in combination, which was confirmed by the lowest abundance of fungal isolates. More fungi were isolated from potato tubers after harvest than after storage, but pathogens were more frequently isolated from stored tubers. After harvest, the lowest number of pathogenic fungi was isolated from the tubers of cv. ‘Adam’ in the non-fertilized treatment, and after storage – from the tubers of the late cultivars in the treatment with three foliar fertilizers applied in combination.


INTRODUCTION
Under conditions of macro-and micronutrient deficiencies in the soil or nutrient uptake problems, multi-component fertilizers applied to potato leaves can almost instantly cure nutritional deficiencies (B ol i g ł o w a , 2003).Foliar micronutrient fertilization affects the yield (B r a r and N a w d e e p -K a u r , 2003) and quality (K o z e r a et al. 2006) of potato tubers.T r e h a n et al. (1995) reported a potato yield increase due to the inhibitory effect of foliar fertilization on late blight.Foliar fertilizers determine potato resistance to pathogenic infections (M i l l s et al. 2006; M a h m o u d , 2007), and influence the species composition of fungal communities colonizing potato tubers (K u r z a w i ń s k a , 1997; C w a l i n a -A mb r o z i a k , 2002).According to the latter authors, Alternaria alternata, Colletotrichum coccodes, Rhizoctonia solani and species of the genus Fusarium are the main causal agents of tuber diseases.
The objective of the present study was to estimate the severity of tuber diseases in three potato cultivars after harvest as affected by mineral soil fertilization and foliar fertilization.The structure of fungal communities colonizing tubers directly after harvest and after five-month storage was also determined.

MATERIALS AND METHODS
Cultivars, medium-early 'Adam', medium-late 'Pasja Pomorska', and late 'Ślęza', were grown in a three-year plot experiment established on grey--brown podsolic soil developed from light silty loam of complex 2, quality class IIIa; the experiment was carried out by the Department of Agrotechnology and Crop Production Management, University of Warmia and Mazury in Olsztyn, in Bałcyny in 2008.Cereal crops were grown as a forecrop.Certified seed potato tubers were planted in rows, 40 cm apart, at a row spacing of 62.5 cm.Tillage treatments and agricultural measures (as recommended by the Institute of Soil Science and Plant Cultivation -National Research Institute, Puławy) as well as the methods of plant protection against agrophages (as recommended by the Institute of Plant Protection -National Research Institute, Poznań) were identical in all experimental plots.The experiment was carried out in a randomized split-plot design, in three replications.

1
), H (control treatment, without foliar fertilization).Mineral fertilizers were applied broadcast, at the same rate in all plots.Foliar fertilizers were applied once, at the beginning of flowering (BBCH 61).The composition of foliar fertilizers (% by weight) was as follows: Basfoliar 12-4-6: N -12, K -6, P -4, Mg -0.2, B -0.02, Mn -0.01, Cu -0.01, Fe -0.01, Zn -0.005, Mo -0.005, ADOB Mn: N -6.5, Mg -2, Mn -10, Solubor DF: B -17.5.The symptoms of soft rot, late blight and dry rot were evaluated in 5 kg potato samples.The results were expressed as the mass percentage of infected tubers.The rates of common scab and black scurf infection were estimated on 100 tubers collected randomly after harvest in 2009 and 2010 according to a nine-point scale (R o z t r o p o w i c z , 1999; 1 -no symptoms, 9 -most severe symptoms) and were presented as a percentage infection index.Laboratory samples consisted of 30 tubers collected randomly in three replications per treatment, directly after harvest (in 2008-2010) and after five-month storage at 5 o C. Blocks (0.5 × 0.5 × 1.5 cm) were cut from the tubers and they were disinfected with 50% ethanol and 1% sodium hypochlorite, washed with distilled water and placed on PDA medium.After seven days of incubation, fungal colonies were inoculated onto agar slants for later microscopic identification according to the relevant keys and monographs (A r x , 1970; E l l i s , 1971; D o m s c h et al. 1980).The results were processed statistically by ANOVA (STATISTICA ® 8.0 2008), and the significance of differences between means was determined by Duncan's test (p = 0.05).
Temperature distribution patterns (May -August) were similar in the investigated growing seasons.Mean monthly temperatures in July and August were higher than the long-term average.Precipitation varied and rainfall totals were more than a half higher in 2009 and 2010, compared with 2008 (Table 1).

CONCLUSIONS
The highest percentage of potato tubers showing the symptoms of soft rot and late blight was observed in the wet growing season of 2010.In 2009 the highest severity of common scab and dry rot was noted on the tubers of the late cultivars, while the incidence of black scurf was highest on the tubers of the medium-early cultivar.After harvest, the severity of tuber diseases varied between treatments with two levels of mineral fertilization and foliar fertilization.Pathogens had a high (over 55%) share in the fungal community colonizing stored tubers of all cultivars after storage and newly-harvested tubers of cv.'Pasja Pomorska'.After harvest, the lowest number of pathogens was isolated from the tubers of cv.'Adam' in the non-fertilized treatment, and after storage -from the tubers of the late cultivars in the treatment where three foliar fertilizers were applied in combination.

Table 3
Percentage mass of tubers infected by Phytophthora infestans

Table 4
Percentage mass of tubers infected by Fusarium spp.

Table 5
Infection of potato tubers by Streptomyces scabies (infection index Ii in %)