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Ditylenchus destructor Tnorne, 1945, the Potato Rot Nematode

It is commonly known as the potato rot nematode because of the nature of damage to the potato tubers. It was first recorded in 188S⁸⁷ followed by reports from U.S.A. as well as some European countries. It is rarely observed to infest above ground plant parts, mainly confining itself to subterranean parts, It is quite a serious problem in some parts of Europe.

Biology

The nematode can survive at temperature as low as —28°C. It can develop and multiply in 60-80 days at temperatures varying between 5-34°C. It is not very tolerant to drought conditions. It appears to thrive very well under U.S.S.R. conditions since about 10-iO per cent potato crop is reported to be damaged by this nema­tode.

    The nematode multiplication is favored by ammonium nitrate fertilization but amide forms of fertilizers depress reproduction⁸". The nematode can be cultured easily on fungi like Aliernaria ienuis, A. solani, etc. and probably, in nature, survives by feeding on the fungi, if host plants are not available,

Symptoms

On potato, the infestation is characterized by production of tiny, white chalky spots. These spots are quite superficial and do not extend deep into the tuber. The nematode may enter the tuber either in the region of eye or through lenticels. As a result of penetra­tion, young developing and already differentiated cells of potato are disrupted. This type of reaction is at variance with the more specialized D. dipsaci which disrupts only differentiated cells with the plant responding back with de-differentiation of cells surrounding the nema­tode. The tiny chalky white spots ultimately increase in size engul­fing large portions of the tissue. The infested area appears granular with a honey combed structure. It is followed by a progressive drying and shrinking of skin, which becomes papery⁸². Apart from the direct effect of the nematode infestation on tubers, the lesions facilitate penetration of soft rot bacteria as well as saprophytic fungi resulting in rotting of tubers. In such cases, a distinct area exists between the decayed portions and the uninvaded tissues and nematode popu­lations are present, in abundance, in such areas.

On iris bulbs, the husk is shredded at the base, with the basal plate and the outer fleshy scale separated by a dark sunken crevice. Yellowish grey or black streaks are present in the fleshy scales and the leaves are distorted. The stem my exhibit light yellow or grey areas attached to the bulbs.

    Symptoms, more or less similar to potato are present in sugar-beets. The nematodes enter the tissues near the crown. Small discolored areas appear below the cortex that may engulf the entire upper part of beet. The infection by secondary micro-organisms results in rotting of tissues. On carrois, hops and Ipontea batata also similar damape symptoms are produced.

    As a result of infection by the nematode on different hosts, there is a decrease in the carbohydrate content of the tissues with increased activities of peroxidase and polyphenol oxidase. Accumu­lation of polyphenol oxidase results in necrotic tissues"⁸. Reduced

amount of starch and increased amounts of amylase and amololytic activity have been recorded in infected potato tubers.

    Because of the polyphagous nature of the nematode and additionally because of its ability to feed on soil fungi, in absence of host, crop rotation is generally not applicable against this nematode. Soil fumigation by ethylene dibromide or granular chemicals like furadan or heteropbos can reduce the populations 'effectively. Heterophos granules applied before planting at the rates of 20 or 50'g/m² are reported to reduce infection as well as increase potato yields.¹⁰³ Hot water treatment of planting materials at 43"C for 10 hours, has been suggested as a practicable control measure.¹⁰¹ Addition of 0.5 per cent formalin in water increases efficacy of hot water treatment. Dry heat treatment can be effective also as has been found in the case of iris bulbs treated at 5;°C for three hours.

In the recent past, attempts are now directed towards finding resistant sources. It has been suggested that forms immune to D. destructo* can be obtained from Solam.m caiarthrum, S. sucrense, S, vernei, S. famaiinae, S. Spega:zim, S. iimplicifclicum, S. acaule, S. chacoense,and  S, demissum.

    On the basis of different approaches employed in the control of the nematode, an integration of all the possible methods has been suggested and is claimed as a viable strategy for control of the nematode on potato. The steps suggested are :

(1) Use of rye or lucerne as the preceding crop;

(2) Winter sowing of rye followed by late spring planting of potato;

(3) Careful sorting of seed potatoes  and  early  harvesting of potatoes ; and

(4) Soil treatment with metham sodium at 250 kg/ha.