Download Signs and symptoms of nematode disease

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.