DISEASES OF CULTIVATED PLANTS 563 



half to two hours. It is then removed and allowed to dry. A longer 

 treatment than two hours will injure the seed. A treatment of one- 

 half is sufficient when the seed is not carrying scab or the fungus of 

 the rosette disease. 



The easier treatment, also the most rapid, is the formaldehyde 

 gas method. It requires a room that may be made reasonably tight 

 by blocking cracks and knot holes. This method is given in the 

 Ohio 'Station Bulletins 199 and 214, from which the writer will quote 

 the formula and operations. 



FORMALDEHYDE GAS. 



Commercial 40 per cent formaldehyde 3 pounds 



Potassium permanganate 23 ounces 



Sufficient for 1000 cu. ft. of space occupied by crates or trays. 



Enclose open tiers or piles of slat crates filled with dry onions, 

 potatoes, etc., in tight room or oiled tent or canvas buried in the 

 earth about the base. Generate the formaldehyde gas in a flat bot- 

 tomed dish or pan of adequate capacity by placing one of the mate- 

 rials, as the liquid formaldehyde, in the pan, and adding the other 

 the last thing before retiring. Then close tight and allow to remain 

 closed 24 to 48 hours. 



Proportionate or multiple unit amounts may be taken for 

 smaller or larger enclosed spaces. Applicable to fumigation of seed 

 potatoes for scab, sweet potatoes for rot troubles and to newly gath- 

 ered, dry onions before storing for winter. (Ohio E. S. B. 229.) 



Potato Scab. The disease first shows itself as a minute reddish 

 or brownish spot on the surface of the tuber. The infested area be- 

 comes more extended forming a scab-like crust which may become 

 cracked and furrowed. Freshly dug, infested potatoes exhioit a very 

 fine, grayish mould which disappears upon exposure to the air. This 

 mould-like appearing matter constitutes the spore-producing portion 

 of the fungus, which Dr. Thaxter named Oospora scabies. (Nevada 

 E. S. B. 26.) 



Preventive Measures for Scab. The most frequent source of 

 infection lies in the seed planted, and the experience of eastern grow- 

 ers has been that by disinfecting the seed and by planting in land 

 free from scab a clean crop may be produced. It is therefore of the 

 greatest importance that those California growers who have land not 

 already infected by this disease should seek to keep it healthy by 

 allowing nothing but disinfected seed to be planted. 



In the majority of cases, however, it is necessary to deal with 

 land that has already been infected by the cultivation of one or more 

 potato crops. The disease has already been introduced ; the problem 

 is, how to control it. It goes without saying that additional reinfec- 

 tion should be avoided by disinfecting all the seed planted, in accord- 

 ance with directions given later. Additional means to be adopted 

 comprise rotation of crops and soil treatment. 



Scab tends to diminish when land is planted in crops -which do 

 not harbor the parasite. Other root crops, however, such as sugar 

 beets, turnips, etc., must be avoided. The length of time required to 

 free land from scab varies in different sections, but, as a rule, a period 



