(I) CITRUS
Citrus is very important commercial fruit crop grown throughout the tropical and subtropical areas of the world. In India. it ranks third after mango and banana among the fruit crops. Over 200 species of plant-parasitic nematodes belonging to more than 58 genera have been detected on citrus roots worldwide. Out of these, approximately 122 species belonging to 57 genera are reported from India. Most important is Citrus nematode, Tylenchulus semipenetrans. Another very important nematode is ‘Radopholus citrophilus’ but fortunately this species is not present in India. The other economically important nematodes are Pratylenchus coffeae, Hoplolaimus indicus and Meloidogyne spp.
a. Citrus nematodes (Tylenchulus semipenetrans)
Causal organism: ‘slow decline of citrus’ / ‘Citrus Decline’/ ‘Citrus Dieback’
- The citrus nematode, T. semipenetrans was first observed on orange tree in Southern California in 1912 by Cobb.
- In India, it was first reported from citrus in Aligarh by Siddiqi (1961).
Distribution
It is a world wide pest of citrus. In India, it is widely distributed in citrus orchards grown in Andhra Pradesh, Assam, Himachal Pradesh, Haryana, Punjab, Rajasthan, U.P., M.P., Bihar, Gujarat and Maharashtra.
Host range
It has a narrow host range as 50 species or hybrids of citrus and plants belonging to Family Rutaceae only are its hosts. Hosts belonging to families other than Rutaceae include grapevine, olive, pear, persimon and loquat.
Biology:
- The first stage juvenile develops within the egg and moult once before hatching as second juvenile.
- Hatching in soil occurs when environmental conditions including temperature, moisture and aeration are satisfactory.
- Eggs hatch in 12 to 14 days at about 24o C.
- The second stage male juveniles develop to maturity within 7 days without feeding and the second stage female juveniles require 14 days to locate the host root, feed and moult.
- The life cycle from egg to egg is completed in 6 to 8 weeks.
Nature of Damage:
- Citrus nematode is a sedentary semi-endoparasite that attacks roots by burrowing its anterior end deep inside the root cortex while the posterior end remains outside in the soil.
- Feeder roots particularly in the upper soil layer are destroyed as nematode prefers to habitat upper soil layers, resulting in reduced uptake of water and minerals from the soil.
- While juveniles remain in superficial layers of the young root tissue, young females penetrate deep in the cortical tissue where they form ‘feeding site’ consisting of 6-10 cortical cells called ‘Nurse Cells’.
Symptoms:
- Initially symptoms are often unnoticed in the seedlings.
- General reduction in tree growth and vigour
- Yellowing and shedding of leaves
- Dieback of twigs and under-sized fruits resembling more or less symptoms of drought and malnutrition.
- Young trees develop slowly and production is delayed.
- In heavily infested roots, soil particles usually adhere with root-lets and finally the cortex separates readily from the vascular stele.
- Heavily infested roots are darker in colour with reduced branch rootlets.
Extent of damage mainly depends upon factors like age and vigor of the tree, density of the nematode population, and susceptibility of the rootstock. Mature trees can tolerate a considerable number of these nematodes before showing lack of vigor and decline symptoms.
If, a heavily infested orchard site is replanted with a susceptible rootstock without soil fumigation, the roots of the young trees are heavily parasitized, tree growth is stunted, and fruit production is reduced. This condition is also referred to as the citrus replant problem.
Citrus plants showing symptoms of Decline
b. The Lesion Nematode (Pratylenchus coffeae)
Pratylenchus coffeae is a highly pathogenic nematode reported from citrus in India, Japan and U.S.A. and is causal organism for ‘Root-rot of Citrus’. In Florida (USA), it is known to cause ‘Citrus slump disease’.
Nature of damage:
- It is a migratory root endoparasite, the adults and juveniles of which invade the root tip and destroy the growing meristem.
- As a result, a lateral root usually initiates near the destroyed root tip.
- The adults and juveniles occur in the cortical tissue of feeder roots where they form small black lesions at their feeding site which expand to girdle the rootlets.
Symptoms:
- Non specific above ground symptoms appear in form of stunted plants, general dieback and undersized fruits.
- Brown black lesions girdle the feeder roots.
- The infected seedlings show marked reduction in root and shoot weight as compared to healthy ones.
c. The Lance Nematode (Hoplolaimus indicus)
This nematode is prevalent in citrus growing sites of Delhi, Punjab, Rajasthan and Uttar Pradesh in India. It has so far not been reported from any where outside India.
Biology:
- The first moult occurs within the egg and the development outside the egg consists of 3 moults giving rise J3, J4 and the adult stages.
- Temperature of 30o C, moisture content of 16% and soil pH of 7.0 are the optimum conditions for multiplication.
Symptoms:
- In orchards nematode infestation show patches of unevenly grown plants.
- Symptoms are visible in 3-4year old sick orchards in the form of plants with reduced height and vigour.
d. Root-knot nematode (Meloidogyne spp.)
Three species of Meloidogyne i.e., M. africana, M. indica, and M. javanica are recorded on citrus from India.
Symptoms and life cycle of the nematode are same as produced on other host plants.
Management:
Following are some of the possible ways which can be adopted to check the nematode populations build up.
- Preventive: Nurseries should never be established on or near old citrus orchards.
- Physical: The bare root treatment at 45o C for 25 min. or 46.7o C for 10 minutes can disinfect seedlings from citrus nematode.
- Cultural: Population of citrus nematode in soil and roots can be reduced by removing the old feeder roots before the growth flush. This practice is followed by application of FYM.
- Application of castor, neem, mahua, mustard or groundnut oil cakes in the tree basin effectively reduces nematode population.
- Irrigation of citrus orchard with sewage water has been found to reduce the nematode population significantly.
- Plant resistance: Varietal screening against citrus nematodes revealed that trifoliate orange (Poncirus trifoliata), Carrizo and Troyer are highly resistant, Smooth lemon Assam, Saoner and Cleopatra mandarin Morrocco are moderately resistant. Hybrid citrus root stocks developed by crossing Rangpur lime (C. limonia) with P. trifoliata viz. CRH-3, CRH-5 and CRH-41 are highly resistant to citrus nematode.
- Chemical control: Application of carbofuran @ 4 and 6 kg a.i./ha, benfurocarb @ 4 and 6 kg a.i./ha or combination of neem cake @ 1 kg/plant with carbofuran reduces nematode population significantly.
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