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SIGNS AND SYMPTOMS OF PLANT-PATHOGENIC NEMATODES Field Appearance The appearance and extent of plant symptoms from plant pathogenic nematodes depends on the species and density of nematodes present, the susceptibility of the crop to those nematodes and environmental conditions. When nematodes are a problem, “target-like” patches of poor plant growth can occur in a field where other plants appear normal. The appearance of patches of symptomatic plants is common for nematodes of all life cycle strategies. These patches may grow in size over time if nematodes are not treated. Damage is variable, ranging from mildly chlorotic (yellowing) areas to patches where plants are severely stunted and plant density is low. Plants in these patches may wilt easily in hot weather. The appearance of the plants often resembles that from many other factors that impact the uptake of water and nutrients such as low areas in which standing water accumulated, were unusually dry, had poor fertility, inappropriate pH, etc. Aboveground Symptoms In most cases, the aboveground portions of plants infected with root-parasitic nematodes may simply appear unthrifty without any notable change in structure. Nematodes may be difficult to diagnose as the cause unless a sample is assayed. Specific symptoms of unthrifty plants that may be associated with nematodes include stunted plants, lost leaves, poor fruit set, small fruit size, wilting during midday, etc. Of course, if the root-parasite is a virus vector the aboveground symptoms will be those related to the virus. More specific symptoms may occur with nematodes that attack aboveground portions of plants. Several nematodes cause chlorosis in leaves which may turn necrotic. Some nematode infections result in altered growth patterns of plant structures. Examples include twisted leaves in onions and alfalfa infested with stem nematodes (Ditylenchus dipsaci), twisted leaves and distorted buds caused by Aphelenchoides in lily and strawberry, and galls in leaves and seeds infected with Anguina. Belowground Symptoms Often the roots of plants infected with root-parasitic nematodes exhibit no change in structure or appearance other than to be smaller than those in unaffected plants. This is particularly true for many migratory ectoparasites. Affected plants may need to be compared to normal plants to recognize that a problem exists. Other symptoms are more characteristic of nematode infection. Lesions of necrotic tissue are a common symptom of several nematodes but most notably for root-lesion nematodes (Pratylenchus spp.) Roots, rhizomes or tap roots may become darkened and discolored and plants may be easily pulled from the ground. Root pruning may also be evident. Necrosis can also occur in bulbs and tubers infected with Ditylenchus dipsaci or D. destructor, respectively. Tissues of infected bulbs may also separate easily. Various nematodes may cause altered growth patterns in roots and other belowground structures. Stubby-root nematodes (Trichodorus and Paratrichodorus) cause the roots to form short, swollen, branched root systems. Root-knot nematodes (Meloidogyne) can cause tap roots to fork so that development of carrots is distorted. Both root-knot and cyst (Heterodera) nematodes can cause extensive root proliferation (hairy-root). Probably the most characteristic symptoms of nematodes are galls produced by root-knot nematodes on roots and tubers. Galls may also form on root tips attacked by some migratory ectoparasites such as dagger (Xiphinema) and needle (Longidorus) nematodes. Representations of the different symptoms expressed by roots are illustrated in Figure 1. Signs of Nematode Disease. Some evidence of nematode infection does not manifest in symptoms and is considered a “sign” of disease. Examples include the presence of nematode “cysts” of Heterodera and Globodera and sand-encrusted roots infected with reniform nematode (Rotylenchulus reniformis) or citrus nematode (Tylenchulus semipenetrans). Figure 1. Types of symptoms that may appear on roots infected with plant-pathogenic nematodes. From Crowe, F. and T. Todd. 1984.