Problems Caused by Plant-Parasitic Nematodes
Loss of Vigor and Yield
Plant-parasitic nematodes vary in their feeding habits; each species causes a slightly different type of damage to the root. These differences are important because they affect the physiology and growth of the plant in different ways. One basic feature of nematode attack, however, is that nutrients and metabolic activities are diverted from normal, healthy growth and fruit production into sustaining the nematode population and repairing the wounds they cause. This is a chronic problem that continues throughout the growing season. All plants tolerate minor attacks by pathogens without a significant impact on vigor, but nematodes become a problem when the population level surpasses the damage threshold; at this point damage is measurable. Because of different feeding habits, reproductive potential, and other factors, the damage threshold is different for different species of nematode. The threshold for damage also will vary according to other variables such as plant age, size, nutritional status, moisture stress, and other disease problems.
Nematodes as Predisposing Agents
Research to determine the precise role of nematodes in plant decline has shown that feeding may have diverse effects on plant physiology. For example, it is clear that some nematodes may predispose fruit trees to other disease problems. Although very little is understood about how nematodes do this, some examples are well documented. Feeding by the ring nematode on peaches, for example, has been linked to increased susceptibility to bacterial canker, reduced winter hardiness, and the development of peach tree short life disease. Other research has shown that nematodes can make plants more susceptible to certain fungal pathogens such as verticillium wilt. The basis for predisposition presumably lies in the disruption of the normal hormonal balance of the root. More research is needed to understand the nature of this interaction.
Nematode-Related Replant Problems
Replanted fruit trees frequently have problems with reestablishment. The cause of these replant problems has been extremely difficult to pinpoint because of the abundance of microorganisms that have been isolated from replant sites. The available evidence indicates that replant disease results from an interaction between nematodes and various microflora. The nematode most frequently associated with this problem is the lesion nematode. The lesion nematode is very destructive; it burrows through the root as it feeds, leaving a trail of dead cells that appear as dark lesions. These wounds allow bacteria and fungi to enter the root. In older, well-established trees, a balance exists between new root growth and the activities of nematodes and microorganisms. When old trees are replaced with seedling trees, however, the balance is upset and the young trees cannot compete with the high populations of pathogens present.
Nematodes as Virus Vectors
Perhaps the most serious nematode-related problem with fruit production in the Cumberland-Shenandoah region is the transmission of plant viruses. The tomato ringspot virus (TmRSV) and tobacco ringspot virus (TbRSV) are both vectored by common species of dagger nematodes (Xiphinema sp.). The nematode acquires the virus when it feeds on an infected plant, then transmits the virus when it feeds on a healthy plant. Figure 2.1 illustrates this exchange. Feeding by dagger nematodes is the only natural means of infection for these viruses. In the absence of dagger nematodes, the virus does not spread naturally from plant to plant. Because dagger nematodes transmit TmRSV, their damage threshold is much lower than that of a parasite that causes damage only by feeding. TmRSV causes serious disease problems in a number of different fruit crops, including peach, apple, plum, cherry, grape, raspberry, and blueberry. TmRSV infects many different plants, including most of the common broadleaf weeds. Good broadleaf weed control may be the most effective way of avoiding these virus problems. TmRSV can be spread in the seed of some weeds such as wind-blown dandelion seed.
Figure 2.1 Tomato Ringspot Virus (TmRSV) transmission