Relative susceptibilities of eleven potato cultivars and breeders' clones toGlobodera pallida pathotype Pa 3, with a discussion of the interpretation of data from pot experiments
Curves according to the equationPf=M′ (1-e−aPi/M′) fitted well to the relations according to Seinhorst's (1993) modelPf=yeyhM(1-e−aPi/M) between initial egg densities of potato cyst nematodes (Globodera rostochiensis, G. pallida)Pi up to 5Th (Th=the tolerance limit of haulm weight) andPf at the end of the growing season (a=maximum rate of reproduction,M, M′=different theoretical maximum egg densities). Variation of estimates ofM′, due to variation of the parameters of the submodelsye andyh for the effect of weight reduction of haulms (and, therefore, of roots) on cyst production and damage to root tissue on egg production, respectively, was small enough to be ignored relative to experimental error. Therefore, ratios of values ofM′, determined in simultaneous pot experiments with different potato cultivars, are reliable measures of the relative host status of these cultivars at initial egg densitiesPi of these nematodes up to about 5Th. Variation between potato cultivars of growth reduction and damage to root tissue by the nematodes reduces the reliability of ratios of rates of relative susceptibility of these cultivars.The ratios between the maximum rates of reproduction ofG. pallida, pathotype Pa 3, on 8 out of 9 cultivars and one breeder's clone of potatoes and this rate on the susceptible cvs Bintje and Irene (relative susceptibilitiesrsa) could be considered to be equal to the ratios of maximum population densitiesM′ on these cultivars (relative susceptibilitiesrsM′) (relative susceptibilities independent of initial egg density). The latter ratios were 0.59 times the first (relative susceptibilities negatively correlated with initial egg density) in one cultivar and one breeder's clone. Relative susceptibilitiesrsa andrsM′ of the tested cultivars and breeders clones suggest the existence of continuous ranges of both relative susceptibilities between 0.50 and 0.15 with, in a great majority of cases,rsa=rsM′.
Resistance and tolerance of potato varieties to potato rot nematode (Ditylenchus destructor) and stem nematode (Ditylenchus dipsaci)
Ditylenchus destructor and Ditylenchus dipsaci are economically important plant-parasitic nematodes, affecting potato production mostly in temperate climates. Management through crop rotation is not feasible because of their wide host range. These nematodes are listed as quarantine pests in many countries. Limited information exists on the resistance and tolerance of currently cultivated potatoes to D. destructor and D. dipsaci. Two greenhouse experiments were conducted to screen 25 potato varieties for resistance to and tolerance for D. destructor and D. dipsaci infections. Reproduction factor (RF) and relative susceptibility (RS) were used to evaluate resistance, while potato tuber damage and tuber weight reduction was used to evaluate tolerance. Based on the RF, 16 varieties were evaluated as susceptible (S) while 5 varieties were evaluated as resistant (R) to D. destructor; varieties ‘Innovator’, ‘Aveka’ and ‘Spunta’ were resistant to D. dipsaci based on RF. ‘Desiree’ was observed to be highly susceptible to D. destructor and D. dipsaci in both experiments and was used as the standard susceptible control variety for the calculation of RS. A scale of 1–9 was used to classify RS of the potato varieties to D. destructor and D. dipsaci, where 9 indicated the highest level of resistance. All classes of resistance to D. destructor and D. dipsaci were observed in the potato varieties tested in the experiments. Six varieties had significantly lower RS to D. dipsaci than the standard susceptible control variety. Tolerant to highly sensitive potato varieties to both nematodes were also observed. RS and external potato tuber damage were identified as suitable methods for resistance and tolerance determination, respectively. This study provides essential information on the status of resistance and tolerance in potato varieties against D. destructor and D. dipsaci but needs to be confirmed under field conditions.
The Yield of Potatoes Resistant To Heterodera Rostochiensis On Infested Land
Yields of a resistant potato variety bred from Solanum tuberosum ssp. andigena were adversely affected when their roots were invaded by H. rostochiensis larvae of a pathotype unable to multiply in them. Yields were inversely proportional to pre-cropping population density. Populations decreased slowly when the resistant variety was grown continuously but showed a tendency to increase after 4-5 years. There was some evidence that pathotypes 1 and 1,2 increased slightly (pathotypes B and C of other workers) whereas pathotype 2 (pathotype A) remained much the same. The dominant gene for resistance from Solanum tuberosum ssp. andigena, that from S. multidissectum and both together appeared to confer some tolerance to the injury caused by root invasion. Yields after methyl bromide fumigation were so increased that it seemed likely that pathogens other than H. rostochiensis were also controlled. Although a significant linear relationship could be fitted to the relationship between the yield of an experimental resistant variety bred from S. tuberosum ssp. andigena and the logarithm of the pre-cropping population density, the real relationship is probably sigmoid.