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′.
Relative susceptibilities of five fodder radish varieties (Raphanus sativus var. Oleiformis) to Meloidogyne chitwoodi
The fodder radish varieties Anaconda, Contra, Defender, Doublet and Terranova, known to have some partial resistance, were compared to the standard variety, Radical, to estimate their relative susceptibility (RS) for both population dynamic parameters of Meloidogyne chitwoodi and to evaluate Pi dependency. This approach must eventually lead to new screening methods for partial resistance tests. Plants were grown under controlled glasshouse conditions. Twelve densities of nematodes in five replications were used. Five plants per 7 l pot were allowed to grow for a period of 11 weeks until their early flowering stage. Few seedlings of all the varieties at Pi=32 and 64 J2 (g dry soil)-1, and all seedlings exposed to the highest density, Pi=128 J2 (g dry soil)-1, died within a week after germination. Replanted seedlings developed into normal plants. Total yield, expressed as total fresh weight, was not affected by M. chitwoodi. A lower percentage of plants with galls was observed on partially resistant varieties as compared with Radical. For Radical, a maximum multiplication rate (a) of 0.38 and a maximum population density (M) of 6.43 J2 (g dry soil)-1 were estimated. Radical proved to be a bad host for M. chitwoodi with all final populations lower than the Pi. The parameter estimates of (M) for Anaconda, Contra, Defender, Doublet and Terranova were 0.011, 0.006, 0.027, 0.020 and 0.009 J2 (g dry soil)-1, respectively. With Radical taken to be 100% susceptible, this resulted in RSM values of 0.17, 0.10, 0.42, 0.32 and 0.14% of these varieties, respectively, reducing high population levels of M. chitwoodi by more than 98%. There was no correlation between the rMgalls and the RSM values, indicating that scoring the number of galled plants will not provide a suitable measure for partial resistance.
Races of the Potato Cyst Nematode in the Andean Region and a New System of Classification
Fifty six populations of the potato cyst nematode collected from Northern Bolivia to Southern Colombia were tested on the European host differentials as well as on six additional clones derived from Solanum vernei and S. spegazzinii in order to determine their potential as differentials for the Andean nematode populations. Although S. vernei (VT n ) 2 62.33.3 was an inefficient host for 32 populations, all differential plants were efficient hosts for most Andean potato cyst nematode populations; thus the expression of variability was limited when British and Dutch differential plants were used. Since there are different systems for classifying races, none of which fitted our findings, a new scheme was developed. Under this scheme British and Dutch races as well as Andean populations of the potato cyst nematode can be identified.