DAMAGE RESEARCH WITH P. PENETRANS IN ASPARAGUS PLANTS.

During cultivation of asparagus plants growth can be inhibited and yield can be reduced by plant-parasitic nematodes. Plant raising companies assume that the root lesion nematode (Pratylenchus penetrans) can cause severe yield loss in asparagus plants. However quantitative information about yield reduction in relation to the degree of infestation of this nematode species in the field is lacking. Research was done in The Netherlands by Applied Plant Research (part of Wageningen University and Research Centre) to determine the maximum degree of yield loss of asparagus plants at high infestation levels of P. penetrans and to establish the height of the tolerance limit for this nematode species. Also was investigated whether a field application of a granular nematicide could prevent or reduce yield loss caused by P. penetrans. Research was done in the field at sandy soils at the PPO location near Vredepeel in The Netherlands over a period of two years. In the first year the most suitable field was selected and on this field different infestation levels of P. penetrans were created. In the second year asparagus was cultivated and plant yield (number and quality of deliverable plants and financial yield) was calculated. At high infestation levels of Pratylenchus penetrans maximum yield loss was 12% (which can mean a financial loss of 7.000 C per ha). Yield started to decrease at very low infestation levels of P. penetrans and no statistical reliable tolerance limit could be calculated. Field application of 40 kg per ha of Vydate 10 G just before sowing of asparagus, could almost completely prevent yield loss caused by P. penetrans. After harvest infestation levels of P. penetrans were much lower than could be expected if asparagus was a non-host for this nematode species. In this paper therefore it is suggested that asparagus plants are (actively) controlling P. penetrans.

Optimization of metam sodium application by rotary spading injection.

In The Netherlands the only chemical alternative for methyl bromide permitted is an application of metam-sodium (MS) with the active ingredient methyl isothiocyanate (MIT). After introduction of a new application method with 'rotary spading injection' legislation restricted the application of MS in 1993 to once in four years and since 2001 once in five years. Efficacy after injection of metam sodium at 10 cm depth and rotary spading a 25 cm soil layer was much better than with shank injection at 19 cm depth with a poor efficacy in the top soil layer. Legislation for arable crops allows a dosage of 300 l Monam (510 g MS/l) per ha. For vegetables, fruits, bulbs and weeds dosages permitted are 600 to 750 l Monam per ha. For an optimal disinfestation result the combination of required dosage and injection depth and rotovation depth had to be established. In a field experiment two dosages Monam were tested for efficacy against Meloidogyne fallax with rotary spading injection at varying injection depths. 300 l Monam per ha was compared with 600 l Monam per ha, both applied at 14 cm injection depth and distributed over a 28 cm soil layer. Another treatment was the application of 600 l Monam per ha, injected at 20 cm depth and rotovated through 40 cm of soil. A carrot crop was sown three weeks after disinfestation and quantity and quality of carrot yield was assessed. Three weeks after application Meloidogyne fallax population was reduced in the furrow with 99% and at 30-50 cm depth with 96% by both applications of 600 l Monam. Efficacy of 300 l Monam was significantly less than 600 l. Nematode population levels after carrot crop stayed lower in the furrow after 600 l in comparison with 300 l Monam. Gross carrot yield was significantly higher after 600 l than after 300 l Monam application at 14 cm depth. Although on this heavy infested field it was not possible to grow carrots without symptoms, after 600 l Monam the percentage of carrots without root-knot symptoms (net yield) was statistically higher than after 300 l Monam.

Soil treatment with hot air (Cultivit) as alternative to methyl bromide.

A new development in physical soil treatment is the application of hot air. Hot air treatment is based on blowing extremely hot air into rotavating humid soil. The method has been developed and applied commercially in Israel for the last few years. An increased growth response (IGR) was observed in several crops like potato, cauliflower, kohlrabi and the flower Esclepia, when the soil was treated with hot air prior to planting. Scientific trials were performed in Israel and Cyprus to quantify IGR and to evaluate the efficacy against plant-parasitic nematodes. Squash was grown in tunnels on root-knot nematodes (Meloidogyne javanica and M. incognita) infested fields in sandy (Israel) and clay loam (Cyprus) soils. In Israel hot air treatment was compared with metam sodium and methyl bromide and a cold air treated control. In Cyprus hot air treatment was compared with untreated control. Hot air treatment increased squash yield in Israel with 90 % and in Cyprus with 150%. Root assessments showed that after hot air treatment the root-knot nematodes were still able to infest plants and cause galling damage. Nematode counts were not reduced by hot air treatment. It may be concluded that the general concept of soil disinfestation is not applicable to hot air treatment. Any positive effect in yield could not be explained by reduction in nematode populations in soil. Possible chemical and biological changes in the hot air treated soils need to be identified. Further research will determine the possibilities and limitations of this method in other crops and under various climatic conditions.

Transmission of tobacco rattle virus (TRV) via seed potatoes.

Susceptibility of potato varieties for tobacco rattle virus (TRV) and sensitivity for spraing in potato tubers both depend on the interaction between cultivar and virus strain. Of the six potato cultivars investigated in this research, cultivar Santana was the most susceptible to TRV, the cultivars Roxy and Saturna were the least susceptible. In general, potato cultivars were most susceptible to the virus type transmitted by P. pachydermus and least for the virus type transmitted by T. primitivus. Potato cultivars showed a large difference in susceptibility for various TRV types. This was the most obvious for cultivar Wilja. Wilja showed high virus concentrations (susceptible) for a virus type transmitted by P. pachydermus but very low concentrations (resistant) for virus types transmitted by P. teres and T. primitivus. Cultivar Santana was the most sensitive for spraing, cultivar Wilja was the least sensitive. As Wilja is very susceptible to the virus strain transmitted by P. pachydermus, but is insensitive for spraing caused by this strain, this cultivar is called a "symptomless carrier" for this type of TRV. All six investigated potato cultivars can pass on TRV via seed potatoes to the next generation (secondary infection) but the degree of this virus transmission depended on potato cultivar, virus type and the interaction between cultivar and TRV and seems to be connected with the susceptibility for TRV. Transmission of the TRV type of P. pachydermus via seed potatoes was the highest in the cultivars Santana and (to a lesser degree) Santé and in cultivar Wilja. Even in the very sensitive cultivar Santana, TRV was passed on via seed potatoes. The prevailing theory that in sensitive potato cultivars TRV particles are immobilized in the necrotic 'spraing' tissue, therefore seems to be inaccurate. Since virus-vector combinations show specific interactions with cultivars, it is recommended to do potato variety research in several fields with different viruliferous trichodorid species.

Comparison of extraction techniques and augers of different size.

Because of differences in winter survival of Pratylenchus penetrans after different host plants, concern arose about traditional extraction and soil sampling techniques. Possible bottlenecks are a too short incubation period of the root material for the time of year, or an auger size to small to pick up tough, fresh, root material. Two experiments were carried out to compare different auger sizes and variations on the standard Oostenbrink elutriation technique with additional filter-incubation of the organic material left on the top sieve (180 microns) of the elutriator. The hypothesis that soil sampling in a green crop results in more root material with a 2.5-cm auger, compared to a 1.3 cm auger, proved to be right. Since there was no effect on the number of P. penetrans recovered, the 1.3 cm auger is preferred because with this auger more cores are taken to gather the same amount of soil, resulting in a better estimation of the population. It appeared that highest yields are accomplished with the standard extraction-plus incubation-method described above. Every methodical effort to improve the extraction effectivity, caused only loss of nematodes. An incubation period of two weeks came out to be minimum. After this period, another 15 to 20% of nematodes could be harvested.