The effect of microbial antagonism and predation contributing to the dissipation in soil of sclerotia of Sclerotium oryzae, the cause of stem rot of rice was investigated. A wet sieving and floatation technique was developed to study the-population of sclerotia in soil. Number of sclerotia was night in soil under wheat or fallow as compared to rice. More sclerotia with greater viability percentage were found in surface soil with gradual decline in numbers at greater depths. Sclerotial population increased if stubbles are not removed but incporated in soil and decreased when soil was turned over irrespective of the removal of stubbles. S. oryzae was found on Echinochloa sp. growing as weed in paddy field, this has not recorded as a host from Pakistan
Micro-organisms were isolated from soil, and infected paddy stubbles. Of the 63 different isolates of fungi 16 of actinomycetes and 11 of bacteria used, S. oryzae was found to be inhibited by Trichoderma hamatum, T. harzianum, Stachybotrys atra, Asperqillus flavipes, A. rugulosus, Penicillium funiclosum, P. rimosus and Bacillus sp. in agar culture. Of these T. hamatum reduced the sclerotial production and parasitized the sclerotia of S. hamatum reduced the sclerotial production and parasitized the sclerotia of S. oryzae. In soil antagonists like A. flavipes, S. atra. T. hamatum, T. harzianum, Strepomyces sp. and Bacillus spp. Did neither eliminate the sclerotial umbers nor result in complete loss in viability. Of these, twostrains of Bacilli were more effective in reducing the viability as compared to fungi or actinomycetes.
Among the artificial fertilizers nitrogen, phosphorus and potassium showed little difference in population of sclerotia in green house experiments. However, none of the sclerotia were viable after 10 days in ammonium sulphate and urea at 1000 and 10,000 ppm whereas at 10 and 100 ppm the viability was reduced by 60% Urea used @ 200 ppm in field Showed an increase in scletrotial viability but no significant difference in Disease Index was noticed. Fungicides viz., Benomyl, Vitavax and Demosan @ 20 & 40 μ g a.i. g-1 soil reduced the population of sclerotia approximately by 50% all of which though remained viable. However, no 5-gnificant difference on stem rot disease Index was noticed
Soil amendments with dried stem and leaf fragment of rice, wheat mustard and Lucerne did not show significant changes in population of sclerotia. Viability of sclerotia after 40 days of amendment with Lucerne at 5% level was less effective. Combined effect of organic amendments and Ammonium sulphate at different moisture levels showed slight changes in sclerotial numbers whereas a considerable loss in viability at 75 and 100% moisture levels was recorded. This effect was more pronounced in soils amended with Lucerne followed with rice wheat. Using oat grains previously colonized by S. oryzae as inculum, the production of sclerotia reatly reduced in soil during early stages of decomposition of mustard, Lucerne, rice and wheat as compared to unamended aoil. At later stages of sclerotia was observed. Volatiles produced during decomposition inhibited germination of sclerotia
Sclerotia imbibed water upto 25-30 mg/1000 sclerotia during 10 hrs which was lost after 20 hrs desiccation. Sclerotia imbibed and lost water after repeateo cycle of rehydration and desiccation, however, sclerotial viability remained unaffected. One of the interesting feature of this work has been the solarization of soil by polyethylene mulch on the complete loss in viability of sclerotia of S. oryzae
Mulching of S. oryzae – infested soil (moist or dry) with polyethylene sheeting during May and June increased soil temperature from 36 C ambient to 48-52 C and resulted in loss of viability of sclerotia. Experiments were preformed at two field locations viz. Karachi and Lahore in Pakistan. Mulching of soil artrificially infested with sclerotia did not reduce numbers but redcuced viability by 100% amendments with Lucerne or wheat straw. Sclerotia 20 cm deep were eliminated after an 8 week period of mulching. Soil naturally infested with sclerotia showed similar effects
In artificially heated soil in ovens, a minimum Two-hour temperature of wet soil at 55 and 60 C reduced the viability of sclerotia to zero; Whereas at 50 and 45 C respectively, 3 and 12-days Two-hours temperature cycle showed such an effect. It is suggested that biological as well as thermal control may be taking place during soil mulching detrimental effects of solarization by polyethene mulching can be applied to the field to eliminate stem rot of rice