DISEASES OF RICE 103 



filter paper over the bottom), the spores may develop, 

 but more frequently only contaminating organisms 

 appear. The most reliable method of isolation is to 

 drop 3 mm. thick slices of a diseased lower node region 

 into a petri dish "containing 1 -per cent of Agar-agar in 

 water (with no nutrient added). This substratum 

 appears to furnish the proper degree of moisture, and is 

 unfavourable to the growth of contaminating organisms. 

 In general the fungus grows poorly in culture and pro- 

 duces spores sparingly, but some strains are exceptional 

 in this regard. When the conidia germinate they 

 quickly produce chlamydospores, which, in contact with 

 any hard substance, assume the character of appressoria. 

 The chlamydospores are long-lived, surviving at least 

 twenty months in a dry condition. Artificial inocula- 

 tions with conidia are very easily made ; the conidia, 

 however, rarely survive over three months. 



Conditions Favouring the Disease. All writers 

 appear to agree that nitrogenous fertilizers strongly 

 predispose to the disease. This appears to have been 

 recognised- in Japan 200 years ago. In America, land 

 that is rested, either dry or under water, or newly 

 cleared land, is especially subject to the disease. This 

 relation has not yet been explained or clearly correlated 

 with any structural character of the rice. 



Certain writers have assumed definite environmental 

 relations of cold, heat, fog, irrigation conditions, etc., 

 but upon this point there is no general agreement. 

 Dry land rice is in general as seriously affected as 

 irrigated rice. 



Relation to Varieties. Varieties vary greatly in 

 their susceptibility to blast, but there is no agreement 

 as to what type of plant is immune. According to 

 Farneti, the loss of resistance in Bertone rice was 

 correlated with some decrease in mechanical and 

 thickening tissue. No variety has yet been proved to 

 be absolutely immune, but some varieties are highly 

 resistant. In America the three leading varieties, Gold, 

 Kiushu, and Honduras, are all highly susceptible, and in 



