Nematodes associated with carrot production in Tasmania, Australia, and the effect of Pratylenchus crenatus on yield and quality of Kuroda-type carrot

A survey for nematodes in carrot crops in northern Tasmania detected lesion nematode (Pratylenchus spp.) in 28 of 33 crops, with P. crenatus occurring most frequently and P neglectus, P thornei, and P penetrans occurring infrequently. Root-knot nematode was detected in five of 33 carrot crops, with Meloidogyne fallax present in two crops. The effect of P. crenatus on yield and quality of Kuroda-type carrot was assessed in 42 plots arranged in a 6 x 7 lattice in each of two commercial crops. High numbers of R crenatus were recovered from fine roots at 58 days after sowing (DAS). However, following formation of the taproot, low numbers were recovered. In both crops, there was a significant negative correlation between numbers of P. crenatus per gram dry weight of root at 58 DAS and the dry weight of root and shoot at 58 DAS. However, the low numbers of P. crenatus/g dry weight of root recovered at subsequent sampling times, and the lack of an increase in the number of P. crenatus in the soil during the season, suggested that Kuroda-type carrot became resistant to P. crenatus following taproot formation. In both crops, P crenatus populations were significantly negatively correlated with plant density and in one crop with average carrot weight. Regression analysis predicted potential yield penalties of 12.1 to 22.3 t/ha between plots with the highest number of R crenatus and those with none. There was no significant correlation between P. crenatus populations during the season and carrot defects. Spatial analysis using Spatial Analysis by Distance Indices determined that P. crenatus in soil were significantly aggregated at all times and in roots at 58 DAS were aggregated in one of two trials, with indices of aggregation (I-a) ranging from 1.27 to 1.64.