SEEDLING-ROT OF RICE IN KIANGSU

WEI CHING-TSAO KUNG HAO PAN REN-SHUI (Nanking Agriculture College)

Seedling-rot is a term applied to all troubles causing death of rice seedlings, involving both non-parasitic and parasitic diseases.The kind of seedling-rot which causes the most damage in the early crop of non-glutinous rice in Northern Kiangsu,where seedling-rot is often serious,is due primarily to physiologic disorders,but it can often be complicated with Saprolegniaceous molds infections. Such disease usually occurs at times with a cold spell accompanied by a period of cloudy or rainy weather.Young seedlings already weakened by unfavorable growth conditions succumb.The use of immature or improperly stored seeds, exposed site or carelessly prepared seed-bed,the use of unfermented green ma- nure,an overgrowth of filamentous algae in the seed bed,too high a temperature during the soaking and sprouting of rice seeds—all these are contributory to the weakening of the seedlings.However,the chief cause seems to be the suffocation of the seedlings which sink into the suspended mud,or are lying free and flat under a deep layer of irrigation water without rooting.Prolonged submergence induces seedling-rot even under a less unfavorable weather condition.This relationship of seedling-rot to suffocation is substantiated by the ready occurrence of the rot in seedlings subjected to conditions leading to poor aeration and by the fact that seedlings exposed to air or submerged under water containing a small amount of hydrogen peroxide (0.03%) are considerably more resistant to low temperatures than those submerged under water,when they are kept at 4-7℃ in a refrigerator. Seedlings weakened by low temperature or partial suffocation may readily be attacked by water molds such as Achlya or other Saprolegniaceous fungi which are but weakly parasitic,killing them while they may otherwise recover when the unfavorable factor is removed. Covering the sprouted rice seeds,after sowing,with charred rice chaff,and controlling the irrigation water to an amount just enough to keep the bed moist, is a measure recommended to combat seedling-rot.The charred chaff keeps the sprout moist but does not prevent aeration which is essential to the well-being of the seedlings.It also keeps away mud and fixes the seeds to a position to faci- litate rooting.Pot test showed that seedlings thus raised have a higher percentage of emergence,though shorter in height than those submerged under the water, and that they are deeper green in color,with a larger number of roots,a higher green weight,and what is more,they offer a greater resistance to low temperatures.How- ever,when a cold wave is forecasted,the field should be temporarily flooded to protect the seedlings,and it will be especially helpful if warmer water from a river or canal is employed.