ESTIMATING CROP LOSS IN ORCHARDS WITH PATCHES OF MATURE CITRUS TREES INFECTED BY TYLENCHULUS SEMIPENETRANS

Two citrus orchards, identified in 1982 as having patches of trees inf ected by Tylenchulus semipenetrans, were used to study the long-term e ffect of the nematode on tree quality. In 1999-93, the distribution of infected trees in both orchards continued to be aggregated. Trees wer e selected in one of the orchards, which had only trace amounts (0-17 juveniles and males/100 cm(3) soil), or high levels (500-9875 juvenile s and males/100 cm(3) soil) of the nematode. Six edaphic and biotic va riables measured in soil from the two groups of trees were not differe nt (P less than or equal to 0.05). Percent silt was slightly higher (3 .0 vs. 1.4%) in soil with trace numbers of nematodes. There were no di fferences between the two groups of trees in variables that reflect lo ng-term damage (tree height, canopy diam, and canopy density). However , fibrous root mass density, leaf area, and fruit yield were 33, 8, an d 23% lower, respectively, on trees heavily infected by the nematode.