ROOT CHOPS. 



507 



I'nr disease is carried aboul with infected tubers. The hitter when fed to 

 cattle will infect the manure. Before planting, seed potatoes should be dis- 

 infected with corrosive sublimate," formalin, or formaldehyde gas. (See 

 page 505). Fertilisers which tend to make the soil alkaline, such as fowl 

 manure, lime, wood ashes, or bone-meal, all tend to increase scabby potatoes. 



A recenl paper by Mr. W. A. Millard. University of Leeds,, states: — 



" tins organism attacks the potato tuber only when the supply 



"f its natural food in the soil becomes exhausted. 



"• This scab is most prevalent on light open soils, where the decay of the 

 organic matter is rapid. It appears to a much smaller degree on heavy 

 -oils, since here the decomposition of organic matter and the growth of the 

 -•abbing organisms are alike retarded by lack of air. Common scab may 

 be prevented by introducing into the soil a sufficient quantity of green 

 plant matter, which acts as a decoy for the scab-producing organisms and 

 protects the potatoes from attack." 



Scab tine iu Rhizoctonia solani. — It is quite common to find on the 

 surface of tubers dark-brown lumps of irregular shape and size, like small 

 lumps of soil, but which become black and show up distinctly in contrast to 

 the potato skin when wetted. They do not adhere very firmly to the skin, 

 and can be scratched off 

 with the ringer-nail or easily 

 rubbed off; when removed 

 they leave very little scar on 

 the skin. This condition 

 is sometimes called Black 

 Speck Scab. Sometimes, 

 however, the lumps may be 

 found to be deeper in the 

 tissues, and even beneath 

 the skin. Each of these 

 lumps is known as a sclero- 

 tium, and consists of a mass 

 of fungus tissue, which is in 

 a resting condition, is cap- 

 able of resisting adverse 

 conditions for long periods, i 



and under favourable conditons gives rise to new growths of hvpha?. When 

 planted with the seed the young shoots may be attacked by these hyphfe 

 developed from the sclerotia, and many plants killed. The stems of young- 

 plants are often rotted round the collar or beneath the soil. Some die from 

 what appears to be a wet rot. 



Other conditions found associated with this fungus are scabbing, 

 resembling that caused by sclerotium, a bunching or rosette appearance of 

 the tops and small potatoes (aerial tubers) formed on the stem above the 

 seat of injury, and sometimes in the axils of the leaves along the stem. 



Besides causing the death of plants, the fungus also produces a rotting of 

 the tubers. This typically consists of a dry brown rot, which extends 

 inwards from the skin, and very much resembles the true rot produced by 

 Late Blight when bacteria are absent. This condition is common in Tas- 

 manian potatoes, and is known as Brown Rust. As indicated above, the 

 sclerotia are the chief agents in spreading the disease. The remedy is 

 selection of seed and the dipping of all seed before planting. The sclerotia 



Tuber showing scab due to Rhizoctonia solani. 

 (The small black nodules are the Bclerotia of the fungus). 



