The pin nematode Paratylenchus spp. and damage in greenhouse lettuce

Pin nematodes, Paratylenchus spp., are small ectoparasites on plant roots that are able to cause damage when present in high numbers. They are known for reducing plant growth in Apiaceae (carrot, celery, fennel, parsley), but have also been reported on mint and turf grass. After finding high numbers of Paratylenchus spp. (up to 3000 specimen/100 ml soil) in commercial greenhouses with reduced lettuce growth, we wanted to investigate if this nematode genus is capable of causing damage to lettuce. A pot experiment was set up to compare lettuce growth in soil naturally infested with 300 Paratylenchus sp./100 ml soil and in similar soil without Paratylenchus infestation. The species could not be determined accurately: morphological features indicated P. nanus and P. bukowinensis, but molecular analysis (18S and 28S rRNA genes, GenBank) pointed to P. dianthus. Pots (1 liter) were planted with a soil-block seedling, as is common practice in commercial lettuce production, and maintained for 10 weeks in the greenhouse at 17°C ( between 9°C and 27 °C). Plant and root growth, as well as nematode numbers, were monitored at two-week intervals by breaking up 3 pots of each treatment. No significant differences between the treatments were observed for any of the plant parameters (plant diameter, number and length of leaves, root weight, length and color of roots, root lesions). An increase in Paratylenchus numbers (about 1455 specimen/100 ml) soil was recorded at the last harvest only. A similar test with transplanted (bare) lettuce seedlings did not show damage either, but a 5-fold increase in Paratylenchus numbers was noted 9 weeks after planting. In another test, lettuce was planted in pots filled with soil infested with 14 increasing densities of Paratylenchus, ranging from 0 up to 1500 Paratylenchus sp./ 100 ml soil. Every infestation level was repeated 5 times. After 10 weeks, root damage was observed at the higher densities: smaller root systems and brown discoloration were visible, but lettuce weight was not affected. However, Pratylenchus spp. were found also in this naturally infested soil (140 /ml soil at the highest initial density), and these might (also) be responsible for the observed root discoloration. Again, Paratylenchus sp. was able to reproduce on lettuce and reached final densities up to 24,000 nematodes/100 ml soil at harvest. Our results indicate that Paratylenchus is not affecting lettuce yield when initial densities are below 1500 specimen/ 100 ml soil. However, the nematode was able to reproduce on lettuce between 8 and 10 weeks after planting and might reach damaging levels in a subsequent lettuce crop. The role of nematodes, especially Paratylenchus and Pratylenchus, in reduced lettuce yields in commercial greenhouses needs more investigation.