PLANT DISEASES 289 



(each mass being known as the mycelium), by means of which 

 they secure their food. Many fungi produce secretions known 

 as enzymes, which break down, or dissolve, the tissues of the 

 plants in which they occur, thus making the food available to 

 the fungus. Plants undergoing such changes generally become 

 diseased. When the fungi reach maturity, they do not produce 

 seeds as do the flowering plants, but small reproductive bodies 

 called spores. The spores are not visible to the naked eye except 

 in large masses ; they are scattered by winds, water, insects, 

 man, and by other animals, and when they fall in favorable places 

 they grow. 



Among the well-known fungous para- 

 sites are those producing rusts and 

 smuts in grain, smut in corn, scab on 

 potatoes, scab on apples, and the brown 

 rot of peaches. Mushrooms, puffballs, 

 and bread mold are common fungous 

 saprophytes. 



378. Diseases controlled wholly or 

 in part by pruning. Pear blight is one Fia l % Bacter ^. hich 



J * produce pear blight 



of the best-known bacterial diseases of 



Magnified about 1000 times. 



plants. It attacks pear, apple, and quince (After Duggar) 



trees. Its symptoms appear as sudden 



wilting and blackening of the blossoms ; after the flowering 

 time the tips of the twigs blacken in from two weeks to a month. 

 The disease spreads to the branches and to all the spurs carrying 

 flower clusters. Sometimes, particularly in pears, the disease 

 travels down into the larger limbs and the trunk of the tree, 

 causing body blight and, eventually, the death of the whole 

 tree. Frequently the fruit is affected and shrivels up. Insects, 

 especially bees, that have come in contact with the bacteria, 

 carry them on their feet, body, and mouth parts to the blos- 

 soms, where the growth and spread of the disease are rapid. 

 The bacteria live over winter in diseased branches which serve 

 as a source of infection the following spring. Pear blight may 

 be controlled by carefully removing and burning all infected 



