Lesion nematodes (Pratylenchus species) are very destructive and leave a trail of dead cells in their path as they feed. These wounds are good sites for secondary infections by fungi and bacteria. Young trees are particularly affected, which helps explain the role of nematodes in replant problems. Fruit trees that experience replant problems include peaches, apples, pears, and cherries. Lesion nematodes have a broad host range and high reproductive potential.
Root-knot nematodes (Meloidogyne species) cause large galls to develop on tree roots (Figure N1.1). A gall forms when a juvenile nematode initiates a feeding site or "giant cell" near the vascular tissues. The cells surrounding the nematode proliferate, and the root swells. The nematode remains at the feeding site throughout its development. After reaching maturity, a single female may release 200 to 500 eggs into the soil. Heavy infections of root-knot on peach cause stunting, loss of yield, and eventual decline.
Ring nematodes (Criconemella species) are relatively easy to identify under the microscope due to thick annulations on their bodies. These nematodes are sluggish and reproduce rapidly. The feeding by ring nematodes can be very damaging, and the problem is especially serious on young trees. These nematodes have been implicated as an important component of the peach tree short life (PTSL) problem. Trees experiencing PTSL usually appear healthy one year and then suddenly collapse and die in early spring the following year. Predisposition to cold injury and bacterial canker are also part of the PTSL complex.
Dagger nematodes (Xiphinema species) are not very damaging to tree roots but are serious threats to fruit production because they are
efficient vectors of certain tree fruit nepoviruses. Viruses transmitted by dagger nematodes include tomato ringspot virus (TmRSV), cherry rasp leaf virus (CRLV), peach rosette mosaic virus (PRMV), and strawberry latent ringspot virus (SLRV).
Several additional plant-parasitic nematodes have routinely been associated with declining orchards, but proof that these nematodes are causal agents of fruit tree disease is lacking and further research is needed. The nematodes most commonly reported include lance nem-atodes (Hoplolaimus species), spiral nematodes (Helicotylencus species), and species of Tylenchorhynchus and Cacopaurus.
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