PLANT DISEASES 289 



to discover other methods for combating the rusts of 

 cereals. 



It was formerly thought that rust spreads most easily 

 in muggy or foggy warm weather, following sunshiny 

 weather. It has been recently found that this is not 

 the case, but that it spreads best in cold cloudy weather 

 during the period between heading out of the grain and 

 harvest time. The reason for this is that the rust 

 spores germinate best and the mycelium also grows best 

 at a low summer temperature. Cool weather also brings 

 abundant dew which furnishes water drops for the spores 

 to grow in. Water may also be furnished by rain or 

 fog, but heavy rains tend to wash off large numbers of 

 spores. Hot weather is unfavorable to the growth of 

 the rust, but after the rust has gained a foothold it may 

 increase the rust damage by drying up the plants more 

 rapidly on account of the wounds in the skin caused by 

 the rust. Since man has no control over the weather 

 conditions, control of the rust is very difficult. From 

 these considerations it is seen that the selection of a 

 proper locality for the cereal is important, since wheat 

 in a low situation is apt to have more moisture and 

 colder night temperature than wheat in high situations. 

 Good drainage is also somewhat important. In irri- 

 gated countries too much water must not be used, but 

 it is a noticeable fact that rust seldom causes damage 

 in irrigated districts. Another feature of good farm- 

 ing which would assist in keeping the rust in check 

 is the rotation of crops, since it would tend to the 

 production of less of one kind of cereal and thus 

 make the travel of the disease from one locality to 

 another less likely. The growing of wheat or any 

 other single cereal crop all over a certain district or 



M. & H. AG. IQ 



