By Larry Duncan and Fernando Alferez
Newly planted citrus trees are far less tolerant of pests and diseases than when mature. Sting nematode (Belonolaimus longicaudatus) was not considered a serious economic threat to Florida citrus until successive hard freezes in the 1980s caused widespread replanting of groves. Patches of stunted, declining trees caused by the irregular distribution of sting nematodes in soil were soon evident in many groves replanted on coarse textured sands (Figures 1 and 2). Although some of the damaged trees eventually largely recovered, they lagged in growth and fruit production by several years compared to unaffected trees.
The arrival of huanglongbing (HLB) in Florida has created a similar, more costly situation. HLB is responsible for continuous, widespread replanting of citrus groves, and damage by sting nematodes is again readily apparent in many young groves.
A recent field trial supported by the Citrus Research and Development Foundation (CRDF) showed that suppression of sting nematodes with nematicides during the first four years after planting increased tree growth and harvested fruit, but probably not enough to eventually repay the costs. (See the Citrus Industry article on the research.) Of course, the trees also suffered from HLB-induced root loss and pronounced fruit drop, reducing their capacity to respond to the management of other root pests and diseases. It is unknown how much differently young trees would respond to sting nematode management if HLB onset were delayed.
It is now clear that individual protective covers (IPCs) prevent HLB until they are removed from young trees. IPCs have been shown to reduce fruit drop and increase root mass, tree size, harvested fruit and fruit quality for at least 18 months (and counting) after IPC removal, despite the rapid acquisition of HLB by the unprotected trees. Given the impediment posed by sting nematodes to grove establishment and the efficacy of IPCs against HLB, the CRDF is supporting an ongoing study to evaluate the responses of IPC-covered and uncovered trees to sting nematodes in plots being treated or not treated with nematicides.
Nine months after planting, covered trees are significantly larger and fewer have died than in plots of uncovered trees. Eventually, the study will show the degree to which young trees without HLB tolerate sting nematodes compared to trees with the disease. It will also show the degree to which trees respond to nematode management if they are initially protected from HLB.
The profitability of IPCs is yet to be determined. However, as their benefits to grove establishment are increasingly understood, growers seeking solutions to serious young tree threats such as sting nematode, phytophthora and root weevils should consider using them in integrated pest management programs.
Larry Duncan is a professor at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Citrus Research and Education Center in Lake Alfred. Fernando Alferez is an assistant professor at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.