One of the major challenges in identifying nematodes as the causal agent of crop damage is the fact that many of them do not produce highly diagnostic symptoms, which are specific and easy to identify. The damage caused by nematodes is often non-specific and easily confused with symptoms of other abiotic or biotic stresses. For example, chlorosis may be due to nitrogen deficiency or may be caused by nematodes; poor stands of growth similarly may be caused by poor soil fertility or moisture stress, or may be due to nematodes.
It is therefore highly recommended to assess for nematodes when crops are suffering yield loss and exhibiting any of the symptoms described below. Additional knowledge on the crop, cropping history, and management practices, combined with information in this guide, may also indicate the possible nematode(s) involved.
Symptoms of nematode damage are found both above and below ground.
Above-ground symptoms fall into two categories: those caused by aerial nematodes attacking foliage and those caused by root nematodes attacking plant roots.
Symptoms caused by aerial nematodes
These are often specific symptoms associated with the nematode pest and therefore may be diagnostic. They include:
• Gall formation, or abnormal swelling of seeds or leaves
• Leaf stripe, bleaching and discoloration of leaves (especially in temperate climates)
• Swollen, crinkled and disorganized tissue growth
• Internal stem necrosis, signified with a red ring
• Inflorescence necrosis
• Chlorosis/browning of leaves (needles in pines) and eventual tree death .
Symptoms caused by root nematodes
Root nematodes almost always cause varying degrees of abnormal above-ground growth, but these symptoms alone are generally not enough to diagnose a root nematode problem. Most symptoms reflect or can be mistaken for other problems, such as reduced water uptake or disturbed mineral absorption. They include:
• Chlorosis (yellowing) or other abnormal coloration of foliage
• Patchy, stunted growth
• Thin or sparse foliage
• Symptoms of water stress, such as wilting or leaf rolling
• Die-back of perennial or woody plants with little or no new foliage
• Reduced fruit and seed size
• Low yields.
Other symptoms that may suggest root nematode infection are:
• Failure to respond normally to fertilizers
• A tendency to react to water stress more rapidly than healthy plants, and slow recovery from wilting
• Little or no new foliage development at the onset of a new growing season
• Severe weed problems (higher density of weeds), due to the nematode-infected plant being less able to compete with weeds
• Greater disease incidence, because of suppressed resistance of nematode-infected plants.
These are due to root nematodes, and may be specific enough to allow diagnosis of the root nematode problem. Uprooting of plants or excavation of roots is needed to observe symptoms. Symptoms include:
• Shortened, stubby or abbreviated roots
• Root lesions
• Root or tuber necrosis, rotting or death
• Root or tuber cracking
• Cysts or ‘pearly’ root
• Deformed roots
• Altered root architecture.
by Shambaza Team