Nematode Pests of Horticultural Crops and their Management (1+1)

Introduction to Symptology

Define Symptom
‘Visible expression or indication of any infection/disease is called symptom’. Symptom may also be defined as ‘the external or internal reactions that occur as a result of infestation by pest/pathogen’.
Significance: Symptoms are important means for diagnosis of a disease and management practices can be undertaken only when the correct information about the disease and its etiology is available.

Kinds of Symptoms:
Non Specific symptoms like poor growth, stunting and discolouration of foliage in patches and wilting are most often the result of nematode infestation but may also be caused by some organisms other than nematodes, nutritional deficiency or soil effects.
Specific Symptoms are better means of diagnosis and are produced by root-knot nematode and most of the above ground nematodes like Anguina tritici, Ditylenchus dipsaci, Aphelenchoides spp. etc.

(A) Above Ground Symptoms
Important above ground symptoms caused by nematodes are:
i. Retarded growth in patches
ii. Increased growth (in a few specific cases)
iii. Discoloration of foliage
iv. Distortion and abnormal growth
v. Temporary wilting

i. Retarded growth

• Slow growth and stunting of plants are common non-specific symptoms of nematode attack. However, stunting of plants when occurs in the patches in field, gives an indication of infestation by nematodes due to their uneven distribution in the field.
• If host crops are grown in continuation, such patches grow in periphery, season after season.
  
• Many nematodes like Pratylenchus penetrans and P. pratensis in crops like cherry, fruit trees and seedlings, Heterodera avenae (in wheat and barley), Globodera rostochiensis (in potatoes), Meliodogyne incognita (in many vegetable and ornamental hosts) cause retardation in plant growth.

Nematode infested patches in tomato field

Patches and stunting in carnation under protected cultivation due to root-knot nematode

• Stunting and dieback in walnut trees is caused by its primary pathogen Pratytenchus vulnus.
• Poor growth and stunting of vines as a general syndrome of decline can be attributed to nematode species like Meloidogyne incognita, M. javanica, Pratylenchus vulnus and P. scribneri

ii. Increased growth

• Nematode infestation though more than often causes growth retardation, in a few very specific cases an increase in growth is also recorded.
• Ditylenchus dipsaci produces multiple crowns in sugar beet and increased tillering in oats (nematode not recorded from India so far).

iii. Discoloration of foliage
Generally indicative of nutritional deficiency, foliage discoloration is also caused by nematodes, thus making it a non specific symptom. Discoloration due to nematode infestation may range from chlorosis, light yellow to deep red, purple or even black and at it best serves only as an indication of possible nematode attack.
A few examples of non specific symptoms of discoloration caused by nematodes are:

• Meloidogyne exigua and Pratylenchus coffeae cause leaf chlorosis in coffee and Ditylenchus dipsaci in Narcissus.

• Premature yellowing of Narcissus leaves can also be due to attack of Pratylenchus spp.
• Premature yellowing of peanut foliage can be due to attack of Mesocriconema spp. and Premature yellowing of cotton foliage can be due to attack of Hoplolaimus coronatus.
• Light green foliage of potato can be indication of the presence of Globodera rostochiensis.
• Pratylenchus zeae causes stunting and yellowing of ginger.

Stunting and Yellowing in ginger due to Pratylenchus zeae

• However, some nematodes cause very specific symptom of chlorosis on their host plants which confirm their infestation. e.g.
  
• White (or pale yellow) tips of rice leaves (about 1 inch of the leaf tip) is due to Aphelenchoides besseyi.
• Clear interveinal discoloration in leaves of strawberry and chrysanthemum producing angulr spots is caused by chrysanthemum foliar nematode.,A. ritzemabosi .
• A fine mottle on the leaves of lemon and orange trees is caused by citrus nematode, Tylenchulus semipenetrans.

Symptom of white tip of rice leaves infested with Aphelenchoides besseyi
Source:

Angular spots on chrysanthemum and Bronzing of strawberry leaves (after Sasser,1971)

iv. Distortion and abnormal growth
Nematodes like Ditylenchus spp., Anguina spp. and Aphelenchoides spp. which feed on the aerial parts (mainly leaves, stem and buds) of the plants cause symptoms of distortion which are generally specific in nature.

Distorted rice ears due to Ditylenchus angustus and Aphelenchoide besseyi respectively

• Foliage distortion in chrysanthemum is caused by Aphelenchoides ritzemabosi.