Comparative study of the resistance of soil samples against root-knot nematodes in tomatoes in organic farming grown outdoors, under polytunnel and in pots.
Tomato landraces and their pests, including plant parasitic root-knot nematodes and their damage were studied and compared to control varieties in a field trial in 2015 and 2016 on two organic farms. The level of infestation was low on the roots of every landraces and control varieties, despite that factors like forecrop and irrigation may have accounted for the accumulation of and damage by root-knot nematodes. We assume a certain suppressive effect of the soil is one of the reasons or the main reason to explain our findings. For further investigations of this phenomenon, a more detailed study was set up in a pot experiment. We used the two types of soil derived from these two farms and a control soil (a mixture of potting soil and sand) in six different treatments (artificially infected and uninfected plants in three soil types), in ten repetitions for each treatment. During the experiment we ensured that the invasive larvae of Meloidogyne species can spread among the pots with irrigation. We extracted the active nematodes from each pot, and observed high numbers of nematodes (including predatory and other, non-herbivorous nematodes) in artificially infected and also in uninfected soils, but damaged roots with galls were found only in the artificially infected pots. The total nematode density was significantly lower in control soils than in the soils derived from the farms. The interesting fact is that in the control soil the damage by root-knot nematodes was higher in the non-infected than in the infected treatment, suggesting that the invasive larvae of Meloidogyne species were able to spread successfully with irrigation water in the case of control soil. The damage however, did not affect the yield, and the weight of fresh shoot and root. These values were influenced more by the type of the soil. Based on our results the soils of the two organic farms show certain resistance to root-knot nematodes, but not the reproduction of the nematodes is inhibited, just the spreading.