THE POTATO IN BRITISH COLUMBIA. 



61 



Early varieties usually escape the disease. Weeds hold moisture around the 

 plants and help to bring about infection. So also does low-lying, undrained, damp 

 land. If the disease has assumed serious proportions in the tops, a certain amount 

 of tuber-infection is inevitable. This may, however, be much increased by careless 

 Jmndiling. The proper way of dealing with a blighted crop depends on circumstances. 

 If the market permits of immediate disposal, the safest plan may be to dig and ship 

 at once. Ordinarily, however, it is best to let the disease die out in the tops before 



Fig. 60. Rhizoctonia on tuber (after Howitt). 



digging, otherwise increased infection of the tubers may result from the disease- 

 spores in the dead tops. After a few days, especially if the weather becomes fine and 

 dry, this danger is lessened. The worst thing possible is to pile the tubers on the 

 field covered with tops newly killed by the disease. This can bring about 100 per 

 cent, infection. It is not advisable to store a diseased crop over winter. Loss is 

 almost certain to occur unless storage conditions are exceptionally good. If storage 

 is to be tried, every care should be taken to remove all mechanically damaged and 

 obviously diseased tubers, and good ventilation should 1 be provided. 



BLACK SCUBP (Rhizoctonia). 



Tubers are often seen with what appear to be 

 small lumps of hard dirt adhering to them. It is 

 found, however, that these are not readily rubbed 

 off, and if moistened they turn black. They vary in 

 size from less than a pin-head to almost that of a 

 5-cent piece. Frequently the layer is so thin that 

 it merely gives a black mottling to the skin. These 

 black masses (Fig. 60) are composed of the resting 

 mycelium of a fungus (Rhizoctonia), and in this 

 stage do no injury. If they are removed e.g., with 

 the finger-nail the underlying skin of the tuber is 

 found to be perfectly sound. If planted with the 

 tuber, however, the fungus develops and may pro- 

 duce considerable injury in the growing crop by 

 attacking the base of the stem near the level of the 

 ground, forming dead brown spots which may com- 

 pletely girdle it. According to the severity of the 

 attack, the vine either dies outright or the supply 

 v\ K . 61. m,i:n,-in,,ia as it affects of food sul.sfan.vs to develop tubers is prevented 



the plants (after Orton). Note from reaching the underground parts from the 



the aerial tubers due to lesions at 



the base of the stem ]<-aves. In such a case the accumulation of food sub- 



