Tuber resistance to P. infestans was evaluated in the progeny of potato genotypes differing in levels of resistance and maturity. Two methods of tuber evaluation were used: testing whole tubers and testing tuber slices.In the progeny, some genotypes were superior in resistance to the more resistant parent. This superiority was never associated with early maturity. The results of both tuber-testing methods were positively correlated but the interaction genotypes x years was more pronounced in the reaction of tuber slices.It is concluded that: (1) transgression in tuber resistance may be obtained, (2) a high level of tuber resistance is difficult to combine with early maturity and (3) the expression of genes for specific resistance was more pronounced in testing tuber slices than in testing whole tubers.A preliminary evaluation of potato families segregating for maturity and tuber resistance revealed that both characters were only weakly correlated (SWIE2YI^SKI et al. 1991). Continued testing of the parents and selected progeny provided new information on tuber resistance and its relation to maturity which is presented in this paper.Three groups of potato genotypes were evaluated: 14 parents and standard cultivars differing in tuber resistance and maturity, 12 progeny genotypes, selected in the first year of testing as outstanding in tuber resistance, and 15 progeny genotypes selected in the first year of testing as early maturing with the highest tuber resistance in this maturity group. Tuber resistance was evaluated, in the autumn, over a period of 3 years (1989)(1990)(1991). Two methods of tuber evaluation were used; testing whole tubers and testing tuber slices. Each genotype was evaluated in the first year in at least 2 replications, with 6 in the second year and 8 in the third year. In each replication 5 whole tubers and 5 tuber slices were tested. The results are presented in a 9-grade scale (9 = resistant). For inoculation, virulent fungus isolates collected from plants infected in nature were used. On Black's differentials their virulence at testing time was: 1.2.3.4.7.10 and 11; sometimes factors 5 and 8 also being expressed.Maturity was determined in plants growing in the field and was measured in days counted from the planting date.The origin of the evaluated potato genotypes and the description of the tuber testing methods are given by SwiE^YlNiSKi et al. (1991).Among parents and standard cultivars there were late maturing genotypes with resistant tubers and early maturing genotypes with susceptible tubers. In this group the correlation U.S.
The reaction to P. infestans in the foliage and tubers of potato was evaluated in resistant genotypes and in their progeny. The genotypes originated from wild species known to carry genes for specific resistance.Evidence was obtained that three types of resistance to P. infestans were presented in the genotypes:(1) Resistance due to major, dominant genes was predominant. The presence of these genes was expressed in both foliage and tubers and did not influence maturity. Some of the genes were R genes, providing specific resistance, but for others this could not be determined as they were effective against the fungus inoculum with the highest virulence available: 1.2.3.4.5.7.10.11. (2) Resistance was present, associated with late maturity. (3) Tuber resistance was present, expressed after inoculation of whole tubers but not after inoculation of tuber slices.Testing of the progeny was essential for the determination of the type of resistance present in the genotypes evaluated.
Available data have been evaluated on the reaction of potato differentials to over 5000 Phytophthora infestans isolates, collected in various parts of the world. The differentials were able to identify up to 11 virulence factors in P. infestans. The isolates differed in virulence expression, depending on the isolate and testing conditions. All 11 virulence factors were found in both ‘old’ and ‘new’P. infestans populations. The resistance of individual differentials was not overcome with equal frequency. The resistance of differentials R5, R8 and R9 was overcome least frequently. This may be due to instability of expression of the respective virulence factors in P. infestans and/or the kind of resistance present in the differentials. Whatever the reason, such resistance may possibly be used in breeding potato cultivars with durable resistance to P. infestans.
A set of 14 aggressive Phythophthora infestans (Mont.) de Bary isolates with a wide range of virulence, was stored for eight years (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) in liquid nitrogen at -196°C (209 samples) and under paraffin oil at 7°C (70 test tubes). The survival rate of samples stored in liquid nitrogen was scored as 88%, 45 days after thawing. The revived isolates were passaged through potato tissues four times and tested for virulence on a set of 11 Black's differentials. The mean number of virulence factors per isolate, of these P. infestans isolates evaluated before storage, was 7.4. The isolates stored in liquid nitrogen and under paraffin oil and passaged four times through potato tissues showed a similar mean number of virulence factors per isolate, 7.3 and 6.9, respectively. Isolates stored under paraffin oil showed no expression of factor avr5, but expression of this factor occurred in six isolates after liquid nitrogen treatment.The initial expression of this factor occurred in four isolates. Before storage, the average aggressiveness of the tested isolates was assessed as 1.7, on a 1-9 scale, where 1 means the most aggressive. After storage and after four subsequent passages through potato tissues, the mean aggressiveness of isolates stored in liquid nitrogen and under paraffin oil reached the level of 1.5 and 2.1, respectively.The mean aggressiveness was not significantly different from the level of initial assessment.
The effect of experiment date, inoculum concentration and number of passages through potato tissue on virulence spectrum and aggressiveness level of four <i>Phvtophthora infestans</i> isolates was examined in four tests. All these experimental variants significantly influenced the expression of virulence. Isolates chosen for study were characterized initially by complex wirulence and high aggresssiveness. The narro west spectrum of virulence was noted atJune and December in comparison with March and September. The inoculum concentration of 200 sporangia in 1 mm<sup>3</sup> influenced stronger frequency of virulence factors, that concentration of 50 sporangia in 1 mm<sup>3</sup>. The cultures passaged many times showed the wider virulence spectrum than single or double passaged isolates. The double passages of pathogen cultures influenced stronger expression factors of virulence than single one. The significant interaction between experiment dates and number of passages and experiment dales and inoculum concentrations were observed. Higher inoculum concentration and more abundant passages on potato tissues positively affected detection of particular virulence factors, especially v5 and v8. The cultures, both double and many times passaged on potato tuber slices, were significantly more aggressive in comparison with single passaged ones. The isolate MP 425 was the most stable in expression of its virulence and aggressiveness. The <i>P.infestans</i> cultures that lost infectivity on rye agar medium can recover their virulence and aggressiveness expression after double and frequent culturing on potato tissues.
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