Corrie H. Schomaker

Estimation of partial resistance in potato genotypes against Meloidogyne chitwoodi

Three new potato genotypes, designated AR 04-4107, AR 04-4096 and AR 04-4098, with resistance towards Meloidogyne chitwoodi, and the susceptible cv. Desiree were grown at a range of population densities of M. chitwoodi in a climate-controlled glasshouse in order to establish the presence and degree of partial resistance. Tuber parts of about 12 g were planted at densities (Pi) of 0, 0.5, 1, 2, 4, 8, 16, 32, 64, 128 and 256 second-stage juveniles (J2) (g dry soil)-1. The plants were allowed to grow for a period of 105 days. Tomato cv. Moneymaker was included and inoculated at Pi = 2 J2 (g soil)-1 to verify the quality of the inoculum by measuring the multiplication rate. Plant height was measured weekly over 11 weeks. At harvest, fresh shoot, root and tuber weights, and number of tubers were measured to express yield. Final population densities (Pf) were calculated as the total number of nematodes found in soil and roots. Tubers were scored for visible symptoms and a root-knot index was calculated. The relation between pre-plant population densities (Pi) and nematode densities at harvest (Pf) was fitted using R. The multiplication rate a of M. chitwoodi on AR 04-4107, AR 04-4096, AR 04-4098 and cv. Desiree was 0.55, 0.27, 0.91 and 32, respectively. Partial resistance rsa of AR 04-4107, AR 04-4096 and AR 04-4098 was 1.7%, 0.8% and 2.8%, respectively. Partial resistance expressed as rsM was 0.2%, 0.2% and 0.1%, respectively. It can be concluded that AR 04-4107, AR 04-4096 and AR 04-4098 are strongly partially resistant to M. chitwoodi. Also, the population dynamics curves run almost parallel between both the tested genotypes and the reference cultivar, indicating that a simple and cheap partial resistance test is feasible. When tuber yields were fitted to the Seinhorst model for yield reduction, cv. Desiree showed a minimum yield (m) of 0.86, while all three resistant genotypes suffered no yields losses at all (m = 1), which indicates that the observed resistance was associated with tolerance. As a result of the remarkably high partial resistance, quality damage was low compared with cv. Desiree. The root-knot index, which takes into account internal quality damage of the potato tuber, was below 10 for all genotypes with partial resistance, the lower damage threshold used for industrial processing of consumption potatoes. Visible symptoms on the tuber skin were absent up to densities of 32 J2 (g soil)-1 for genotypes AR 04-4098 and AR 04-4096 and 2 J2 (g soil)-1 for AR 04-4107, and significantly reduced at higher densities when compared with the susceptible cv. Desiree. However, when tuber peels were investigated, egg masses were detected in tubers at almost all initial population densities.

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.

Ways to improve the accuracy of hatching tests for Globodera spp. with depecial emphasis on nematicide trials.

Hatching tests are laborious and yield variable results. In this paper, sources of variability have been identified and analysed, and solutions are presented. A method was developed to conduct hatching tests using inert materials to minimise the total variation at the end of the test. Hatching tests were carried out to increase reliability, optimise the method and limit the amount of work. Thus, it was possible to obtain a coefficient of variation (cv) of the hatching process, which was in accordance with the combined errors expected when a certain number of cysts are treated and eggs are used. An Appendix is provided listing the different errors and ways to calculate and cope with them. The results indicate that the hatching process is no longer an important source of variation for the end result. All variation greater than expected could be explained by variation between batches with the same treatment, indicating that small differences in nematicide application cause major differences in the end result. The treatment effect was more important in field experiments than in laboratory experiments. The hatching curve could be described adequately by a log-logistic curve with three parameters (λ, final number of hatched juveniles; α, time; t when cumulative hatch equals 0.5 × λ; β, slope parameter). Addition of a fourth parameter (γ, incubation time) improved the fit significantly. Using the log-logistic model, final hatch could be predicted with a certain error, but in general final hatch was underestimated. When an error of 5% was accepted, the duration of hatching tests using laboratory reared cysts could be reduced by 80% for untreated batches and by 40 to 80% for batches treated with nematicides. The acceptable reduction in hatching test duration was negatively correlated with the concentration of the fumigant used. Hatching tests with cysts originating from field experiments are unsuitable for prediction using a time-limited data set. Compound hatching curves were distinguished in four of six fields, indicating that the soil samples contained at least two proportions of cysts with different hatching responses. Prediction would cause a significant underestimation of final hatch and consequently an overestimation of mortality.

Damage thresholds and population dynamics of Meloidogyne chitwoodi on carrot (Daucus carota) at different seed densities

for sedentary and free-living nematodes. The tolerance limits for yield loss were 0.34, 0.62 and 0.50, while that of quality were 0.012, 0.142 and 0.813 second-stage juveniles (J2) (g dry soil) −1 at increasing seed densities, respectively. The minimum yield (m), increased with seed density: 0.25, 0.30 and 0.50; the minimum quality yield was 0.10, 0.08 and 0.15 J2 (g dry soil) −1 at increasing seed densities, respectively. Both maximum multiplication rates and maximum population densities increased with increasing seed density but were generally low. Carrot cv. Nerac can be considered a bad host for M. chitwoodi.

Tuber and root resistance of potato genotypes against Meloidogyne chitwoodi in the presence of Avena strigosa, related to tuber quality

Relative tuber infestation and quality of two Meloidogyne chitwoodi resistant potato genotypes, AR04-4096 and 2011M1, were compared in glasshouse experiments at initial population density (Pi) = 16 second-stage juveniles (g dry soil)−1 in the presence and absence of the bristle oat, Avena strigosa. When A. strigosa was added, Pfroot+soil (Pf= final population) on both AR04-4096 and 2011M1 increased 130×, Pftuber increased 1.9 and 3.7×, respectively, while Pftuber × fresh root weight (FRW)−1 was the same. Nematode hatch from peel of AR04-4096, without A. strigosa, was delayed by 3 weeks but relative hatching rate was increased. Although the RStuber (RS = Relative Susceptibility) of both AR04-4096 and 2011M1 were lower than 1%, in the presence of A. strigosa tuber quality of 2011M1 dropped below the marketable level, while that of AR04-4096 was hardly affected. We conclude that: i) Pftuber is influenced by root mass; ii) root quality influences nematode hatch; iii) tuber quality is not an estimator for tuber resistance, and the reverse; iv) root resistance is equal to tuber resistance.

A routine test for the relative susceptibility of potato genotypes with resistance to Meloidogyne chitwoodi

The population dynamics of Meloidogyne chitwoodi on eight potato genotypes was compared to the susceptible cv. Desiree in four glasshouse experiments. The initial nematode densities consisted of log series 2x , with −4 < x < 8. Seinhorst’s logistic model was fitted to the final population densities to estimate the parameters maximum multiplication rate (a), maximum population density (M) and the ratios RSa, RSM and M/a. Average RSa and RSM of the seven resistant genotypes were smaller than 0.29%. The M/a ratios on six resistant genotypes and cv. Desiree were the same, 1.3, indicating Pi independence of RS. One genotype stood out with M/a = 8.6, whereby RSa < RSM. Both RS and M/a were unaffected by pot size or experimental conditions. Screening protocols at Pi = 24 second-stage juveniles (g dry soil)−1 in 2 or 3 kg pots were evaluated for distinctiveness between the two genotype groups. Based on the results, an optimal protocol for a routine resistance test is proposed.

The effect of storage time and temperature on the population dynamics and vitality of Meloidogyne chitwoodi in potato tubers

The population densities of Meloidogyne chitwoodi in potato tubers stored at 4, 8 and 12°C after 0, 60, 120, 180 and 240 days of storage were assessed. Compared to day 0, storage temperatures of 4 and 8°C reduced population densities to 9 and 35%, respectively, after 240 days of storage, while nematode numbers in tubers stored at 12°C increased 2.5 times. The maximum hatching rate of nematodes from tubers stored at 8 and 12°C increased linearly with storage time. At 4°C it remained constant. The time required for the hatching process to reach the maximum number of second-stage juveniles (J2) decreased with increasing storage temperature. Recovered juveniles of M. chitwoodi from tubers after 180 and 240 days of storage at all three temperatures were still infective and able to multiply on ‘Desiree’ with estimates of the maximum multiplication rate ( a ) and the maximum population density ( M ) of 63.6 and 70.8 J2 (g dry soil) −1 , respectively.