PLANT AGE INFLUENCES DAMAGE SUSTAINED BY NEMATODES
Antoon Ploeg, Department of Nematology, UC Riverside
Field observations on the susceptibility of crops to nematode damage suggest that the earlier the nematodes attack a plant, the more severe the damage. In spite of these observations, there has been little research on the effect of plant age at the time of nematode attack and resulting yield loss. Systemic nematicides reportedly do not kill all the nematodes, but temporarily prevent them from reaching the plant roots. Thus, the effects of such nematicides are probably partly due to allowing very young plants to grow without nematodes.
One greenhouse study on plant growth and timing of nematode attack showed, that allowing oat plants to grow for 3 weeks before adding cereal cyst nematodes to plants resulted in much larger plants at harvest. However, the number of nematodes at which a reduction in plant growth was first observed, was not different for plants that were inoculated at seeding or much later. Thus, plant growth started to become affected at similar nematode densities, but the ultimate damage was much more severe when the nematodes attacked very young plants.
A number of alternative nematode control strategies also rely on temporarily reducing the nematode densities around the plant roots. For example, soil solarization can reduce nematodes in the upper soil layer, allowing plants to grow for some time before nematodes from deeper soil layers migrate to the growing roots. It is, therefore, important to know how long roots need to be protected from nematodes in order to avoid serious damage.
We have studied the effect of delaying nematode attack for three different nematode-crop combinations: melon and the root-knot nematode Meloidogyne incognita, and lettuce and carrot and the free-living needle nematode Longidorus africanus. The experiments were done in pots in the greenhouse. The plants were seeded at weekly intervals, and plants of different ages were then inoculated with a range of nematode densities. Eight weeks after inoculation the experiments were stopped and data were collected.
The results showed that delaying nematode infestation had a very strong effect on the final plant weight. For example, the growth of melon plants that were inoculated at seeding, with 100 root-knot juveniles per 100 g soil, was reduced by over 90%. In contrast, growth of inoculated and seeded simultaneously survived. Plants that were 2 or 4 weeks old at inoculation, however, still yielded ca. 70% compared to the non-inoculated controls, even at very high (>1000 juveniles/100 g soil) inoculum densities.
Figure 1. Growth of melon plants 8 weeks after inoculation with root-knot nematodes Meloidogyne incognita. Plants were inoculated at seeding, or 2 or 4 weeks after seeding.
The severe damage to the melons that were inoculated at seeding was also reflected in the nematode densities at harvest. Plants which had been inoculated with 100 or more juveniles, resulting in very small plants or plant death, had very few or no nematodes recovered at harvest. However, the roots of plants which had been inoculated 2 or 4 weeks after seeding yielded very high nematode numbers at harvest. Thus, allowing melons to grow for at least 2 weeks before the nematodes attack the roots, resulted in much less damage, especially when nematode numbers were high, but at the same time resulted in much higher nematode densities at harvest.
Similar results were obtained with lettuce or carrot and the free-living needle nematode Longidorus africanus. Delaying nematode until seedlings were 2 weeks old, resulted in much less damage at harvest.
So far, all experiments on the effects of delaying the nematode infestation of plants on plant growth have been done in pots in greenhouses. We plan to test whether the strong effects that we observed in these pot experiments, also occur under field conditions.
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