By Megan Dewdney, Tripti Vashisth and Lauren Diepenbrock
Dead wood has long been challenging for fresh citrus fruit production. Spores in twigs can form which cause blemishing diseases that downgrade fruit for lower profits. More recently, it has come to light that there are horticultural and entomological reasons to remove this wood as well.
Many growers are familiar with melanose, the disease most widely associated with dead wood. The fungal spores formed in the fine dead twigs (less than 1/8-inch) blemish the leaves, twigs and young fruit. These spores are most damaging to grapefruit, but if there is significant dead wood from any cause, oranges and tangerines can be damaged, too.
Other diseases can also take advantage of these twigs. Citrus black spot (CBS) is another disease that has spores formed in the dead wood in the canopy, causing lesions on fruit. These spores are less abundant than those from melanose, but since CBS is a quarantine disease, removing as much inoculum as possible is wise to improve disease management. CBS is most problematic on late hanging sweet oranges but is known to infect nearly all citrus types. Although unconfirmed, it appears that some of the inoculum for diplodia stem-end rot comes from dead wood. These diseases are three good reasons to consider removing some of that wood.
Damaged or dead wood can provide habitat for unwanted insect pests. In recent years, there have been instances of very small ambrosia beetles and much larger cerambycid and buprestid beetles taking advantage of this wood. These insects are generally not problematic in healthy trees. However, populations can build up in dead wood and spill over onto healthy trees.
In one recent example, a grower experienced branches loaded with ripening fruit breaking and dropping off, losing those fruit from his harvest. Upon further inspection, larvae were found from a wood-boring beetle in these branches. In this case, the eggs had been laid on dead branches, and when the larvae hatched, they tunneled from the dead branches to the intersection with the healthy branches, weakening that junction and causing the branches to break in the wind. Prevention of unnecessary fruit loss from boring beetles is another reason to consider removing dead wood.
In addition to harboring pests and pathogens, dead wood is wasted space where fruiting wood can grow. Often you can see these dead twigs/branches in the outer canopy where significant sunlight reaches, therefore good potential of fruiting is lost. Pruning this wood along with a good nutrition program can rejuvenate trees. HLB-affected trees have lots of dead branches. In a pruning trial, light hedging of HLB-affected trees with constant supply of nutrients improved canopy and root density compared to unhedged trees.
Dead wood can also intercept with foliar sprays or fertilizer distribution, so removing it is advantageous. While selective removal is ideal, it is expensive and time consuming. Therefore, light hedging is an acceptable alternative.
Megan Dewdney is an associate professor and Tripti Vashisth and Lauren Diepenbrock are assistant professors — all at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.