CONTRIBUTIONS FROM THE WISLEY LABORATORY. 



549 



a foot high, some of each group of pots were over-watered and kept 

 over- watered for a few days. The effect of this was very marked. 

 Those in the uninoculated soil were shghtly checked in growth, but 

 with proper treatment subsequently as regards watering, they 

 recovered. Seventy-five per cent, of those in the inoculated soil, on 

 the contrary, gradually sickened and died, although treated in precisely 

 the same w,ay as the others, and the fungus, Thielavia hasicola, was 

 found on the roots, and in some cases, also on the part of the stem 

 just above the soil surface, of all, but not on those which recovered, 

 nor on those in the uninoculated soil. It would therefore appear that 

 the weakening of the roots by the over-watering laid the plants open to 

 the attack of the fungus, which but for that would have been harmless. 



Thielavia hasicola is capable of growing on dead vegetable matter, 

 and can therefore no doubt maintain itself for a considerable time in 

 a soil which contains organic matter of this nature. It is also, as 

 we have pointed out; capable of growing on a variety of different 

 plants. It is therefore improbable that rotation of crops would be by 

 any means a certain mode of avoiding the presence of the fungus. 

 Indeed we have frequently heard of sweet peas suffering severely from 

 attacks when planted in newly-turned up pasture land, or in land that 

 had lon^ carried other crops. 



The partial sterilization of the soil by steam has proved most suc- 

 cessful in America in dealing with the disease on tobacco, but that is 

 practically impossible to effect on a large scale outdoors ; and it is ques- 

 tionable whether soil-sterilization would be altogether advisable with 

 peas, for it would prevent the formation of nodules upon their roots. 

 Probably a thorough soaking of the soil with a solution of one part of 

 formalin in two hundred of water would also affect sterilization to a 

 suf&cient extent, and if it were attempted the solution should be applied 

 at least three weeks before the sweet peas are put out or the seed is 

 sown. 



The N^ional Sweet Pea Society collected information regarding 

 the outbreaks of the disease from many of their members in 1910,* 

 but nothing in those reports showed that any one variety or type of 

 sweet pea was more susceptible than another, nor that the disease 

 was more prevalent on one type of soil than on another,, or with one 

 system of manuring than another. In fact, the answers given failed 

 to show that any single predisposing condition obtained through all 

 the attacks. From our own experiments, however, and from a con- 

 sideration of attacks examined in various gardens, we have to conclude 

 that any cause tending to the weakening of the root of the sweet pea 

 will lay it open to the attack of the fungus. 



At first the almost const,ant presence of free-living eelworms about 

 the roots of affected plants suggested that they were a cause of the 

 necessary weakening, and it is quite possible that they may be, though 

 the evidence available is not very conclusive. A frequent cause is 



* See Sweet Pea Annual, 1911. 



