A Fundecitrus study found that the first four stages of orange tree shoot development are the ones that should be most protected against HLB-spreading psyllids. The Brazilian association reported that among the six stages of shoot development, the first four (V1 to V4) are the most attractive to the psyllid and favorable for its feeding and reproduction. Therefore, those are the stages that should be most protected with insecticides.
The purpose of the study was to identify the ideal application interval on vegetative orange shoots, which are attractive to the psyllid, in order to avoid new HLB infections. For the study, the shoots received a fluorescent dye, which simulates the presence of insecticides, and were photographed weekly in a growth chamber for 14 days, under spring and summer conditions.
It was observed that the stages V1 to V4 develop rapidly, and that the expansion of the leaves renders new tissue unprotected. In those early stages, the shoots were vulnerable to the arrival of the psyllid and provided “windows for infection.” At this stage, spraying at intervals longer than seven days will not be effective in preventing the transmission of the HLB bacteria.
“This is because the insecticides applied to the leaves do not have a systemic action, that is, they do not move within the plant and do not follow the growth of the shoots,” said Juan Arenas, the postdoctoral fellow responsible for the work.
But in the V5 stage, there is no expansion and only ripening of the leaves, so the coverage of the product remains intact, and applications can be made every 14 days. In this case, the application interval will depend on the occurrence of rain (which removes the insecticides) or on the degradation of the product (which reduces its residual period).