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The Yellow Ear Rot Disease

Corynebacterium Michiganenes PV, tritici (Hutchinson (Dye & Kemp (C.m. pv. tritici), present in the nematode galls, is the primary source of inoculum for the expression of the bacterial symptoms. It is recorded that about 40-55 per cent of the nematode galls carry the bacterium²³. However, it does not mean that some galls defini­tely do not carry the bacterium. The non- isolation of the bacterium from some of the galls could be either due to absence or limitations of the techniques employed in the studies. In an experiment, where surface sterilized larvae were used for inoculations of wheat seedlings, the nematode galls produced did not show presence of the bacterium, irrespective of autoclaved or non-autoclaved soil used in the experi­ments. An obligate etiological relationship between the nematode and the bacterium has been suggested^23-35. In a later study, a positive association of C.michiganene PV tritici with nematode larvae has been

The type of the inoculum as well as the soil, determines the symptom expression of tundu or ear-cockle. It has been established that if unsterilized larvae are used for inoculations in unsterilized soil, maximum expression is for ear-cockle (74.6 per cent) with a mean number of galls as 135. However, with unsterilized larvae in sterilized soil, there is a comparative increase in tundu expression (20.5 per cent as against 2.0 per cent), and it was suggested that this could be because of the possibility that the bacterial colonies on the surface body of larvae are protected from interference from other soil microflora. With the use of unsterilized galls as the inoculum source, the percentages for ear-cockle and tundu symptom expression were 67 and 18 respectively²³.

In an attempt to understand the physiological basis of asso­ciation of the bacterium and the nematode, nutritional requirements of the bacterium were evaluated. It was found that the gall extract contained all the essential amino acids (hystidine, glycine, aspartic acid and glutamic acid, which are required for the growth of the bacterium) and it was suggested that for the growth of the bacterium, the nematode galls contained adequate nutritional requirements¹⁷.

Control

Nematode galls are the primary source of perpetuation of the disease and therefore, cleaning the seed lot, from nematode galls, is the most effective method of control. For this, mechanical cleaning or soaking the contaminated seed lot in 20 per cent brine solution, which allows the nematode galls to float up and thus can be skimmed off, can be followed. Care has to be taken to wash the seed lot with

plain water 2-3 times after brine treatment, to remove the adhering salt particles, as otherwise seed germination is impaired. The brine solution is prepared by dissolving 40 Ibs of sodium chloride (common salt) + a small quantity of potassium chloride in about 60 liters of water. The contaminated seed in the brine solution are stirred vigorously and frequently and the floating galls are skimmed off.

Mechanical separators have been devised also and have been used with success (99 per cent) in China.

At harvest, some of the galls may fall to the soil and may carry the infestation in the next season. Therefore, rotation of field may help also. Under tropical humid conditions, the galls may rot away in the soil. Therefore, seed contamination remains the main source of infection. Some of the bacterial flora like Bacillus subtilis, B. pumilis B. cercus, etc., obtained from some nematode galls is re­ported to be larvicidal. Wheat seed treated with the suspension of these bacterial organisms have shown very little infestation by the nematode¹⁰.