176 PRINCIPLES OF PLANT CULTURE 



mature later. It is generally important to cut the dis- 

 eased branch some distance below the point of visible 

 infection, as in many cases the parasitic invasion extends 

 farther than external appearances indicate. 



324. Infection may be prevented best as a rule by 

 destroying the spores through the use of chemicals (fungi- 

 cides) or by heat. According to the nature of the parasite 

 this may be accomplished either by soil treatment, seed 

 treatment or treatment of the aerial parts of the plant. 



325. Soil treatment is necessary in the case of certain 

 organisms that live over in the soil and attack the young 

 plant as it germinates. The organism causing club 

 root (Plasmodiophora brassicae) of cabbage persists a 

 long time in soil that is slightly acid, but by adding lime, 

 75 to 120 bushels to the acre, to render it alkaline the 

 organism is destroyed. Onion smut ( Urocystis cepulae) 

 also lives over in the soil, but by applying sulfur, or better, 

 formalin solution (2 ounces in 1 gallon water) in the drill 

 along with the seed, infection is prevented. 



The most serious trouble with seedlings in general is 

 caused by the damping-off diseases ^ caused by fungi 

 which live in the soil and invade the young stems and 

 cause them to rot off at the surface of the ground. 



326. Soil sterilization is the most effective remedy 

 for damping-off fungi. Sometimes chemicals, especially 

 formaldehyde, can be used for this purpose but in general 

 heat is the best agent. The simple way sometimes em- 

 ployed of generating the heat is to burn brush on the spot 

 to be used as a seed-bed. The most effective and widely 

 applicable measure is steam sterilization. For this pur- 



^ There are a number of fungi capable of causing damping-off 

 of plants in the seed beds ; the most common of these are species 

 of Pythium, Rhizoctonia, Fusarium and Thielavia. 



