Use Physical Barriers for Root Health

Jim Rogers IPCs, Root health, Tip of the Week

By Larry Duncan

Citrus trees in Florida soils infested with diaprepes root weevil (Diaprepes abbreviatus) or sting nematode (Belonolaimus longicaudatus) frequently contend with devastating damage to their root systems. This is made even worse by root loss due to huanglongbing (HLB) disease.

Where soil conditions are conducive to the insect or nematode, it is very difficult to prevent major economic loss, especially in newly planted trees. Two kinds of physical barriers are increasingly important components of integrated management programs aimed at these pests. Landscape fabric applied as a soil mulch and individual protective tree covers (IPCs) on young trees each function by interrupting insect life cycles — those of the root weevil and the psyllid vector of HLB.

LANDSCAPE FABRIC

Landscape fabric blocks the newly hatched weevils from entering the soil where they would ordinarily grow to adults by feeding on progressively larger roots, causing irreparable loss of root cortex and providing entry for yet other pathogens, Phytophthora nicotiannae or P. palmivora.

Root Health
Figure 1. Young lemon trees were planted with landscape fabric to block diaprepes root weevil larvae from entering the soil (left). The fabric was installed prior to planting (right). The inset photo shows the size of the newly hatched larvae compared to the openings in the fabric.

The fabric can be installed mechanically (Figure 1) or by hand and performs best when coverage extends as far as possible (5 feet or more) on either side of the trunk. It is critically important to use a fabric such as Lumite, which allows good water infiltration. Long-term savings on costs of herbicides and insecticides help defray the high cost of fabric installation.

INDIVIDUAL PROTECTIVE COVERS

Recent research confirmed the efficacy of IPCs for HLB prevention, and ongoing trials have shown that they protect growing trees from weevils in heavily infested groves (Figure 2). Thus, young trees are spared damage by both insects and respond accordingly.

Root Health
Figure 2. Waist-high trees show roots damaged by HLB and diaprepes root weevil (left); chin-high trees have roots undamaged by HLB or diaprepes root weevil (right). Neither psyllids nor weevils have access to foliage with individual protective covers.

Citrus trees also photosynthesize and grow faster when covered, even in the absence of pests. Ongoing field trials will test whether these healthier trees better tolerate the root damage by sting nematodes and aid the performance of nematicides in the absence of greening. Managing diseases and insects such as lebbeck mealybug that can occur inside IPCs represents a challenge to their use, but one that can be overcome with close attention and appropriate treatment.

Larry Duncan is a professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.

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