CHAPTER XXIX 

 PLANT DISEASES 



DISEASES of plants are chiefly caused by fungous growth, but 

 some very bad diseases, such as apple and pear blight, are caused 

 by bacteria. The causes of a few very destructive diseases, such 

 as peach yellows, have not been determined, as to the exact germ. 



A fungous disease is usually easier to control than others. Its 

 work is more or less noticeable, particularly during the advanced 

 stages; the cause of such a disease is more easily determined and 

 more easily prevented. 



Fungi (fun-ji, plural of fungus) are real plants but they are 

 propagated by spores instead of by true seeds. There are two 

 very different types of fungi: (1) Those which live on dead or 

 dying organic matter, such as mushrooms and molds, called sap- 

 rophytes; (2) those which live on the tissues of living organisms, 

 such as wheat rust, apple scab, oat smut and many others. These 

 are called parasites. Parasitic fungi cause much damage to farm, 

 garden, and orchard crops. 



Potato scab (Fig. 286) is a fungous disease which lives in the 

 soil where diseased potatoes have grown and also on the tubers 

 themselves. The disease is harmful in reducing the yield and in 

 rendering the crop less salable. The tubers are actually much less 

 valuable for use because of the deep " scabs" or " sores" in the 

 surface, which cause much waste when they are being prepared 

 for the table. 



As the spores of this disease live in the soil, as well as upon 

 the seed potatoes, great precaution should be exercised to avoid 

 introducing the spores into a garden or field. This may be pre- 

 vented by treating the seed potatoes to kill the spores before plant- 

 ing time. The simplest treatment is to soak the potatoes for about 

 two hours in a mixture of formalin and water, using one pint of 

 full strength (40 per cent) formalin to 32 gallons of water. The 

 mixture may be used a number of times in the same day, and as 

 the material is inexpensive the cost of the treatment is very slight. 



A number of other treatments are sometimes used, such as 

 copper-sulfate solution and dusting with sulfur. 



t is of little value to treat the seed potatoes if the disease is 



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