MARKET DISEASES OF VEGETABLES. 11 



However they generally enter plant tissues through wounds 

 or abrasions of the protective layers. This type of infection 

 is the most important in transportational diseases. Injured 

 or dead areas offer an 'excellent foothold for rot-producing 

 organisms. 



Diseases Due to Non-living, Non-parasitic Causes. 



A very considerable number of plant diseases are due not 

 to parasitic organisms but to other unfavorable factors in 

 the environment of the plants. Water core and Jonathan 

 spot of apples, black heart, hollow heart, and sunburn of 

 potatoes, tip-burn of lettuce and other crops, and scald and 

 freezing or chilling injury of vegetables and fruits are excel- 

 lent examples of disease due to non-parasitic causes. 



Tip-burn of lettuce and other plants, water core and 

 Jonathan spot of apples, and blossom-end rot of tomatoes 

 and watermelons probably are due to irregularities in the 

 water relations of these crops. The nature of the plants 

 themselves is also a very important factor in such troubles. 

 For example, with the same soil and climatic conditions, the 

 same water supply, and the same treatment of trees of vari- 

 ous varieties, the Jonathan and the 'Grimes Golden alone 

 may show Jonathan spot. Black heart of potatoes is a good 

 example of the evil effects of disturbed respiration in plants. 



Under normal conditions, oxygen enters the internal 

 tissues of the potato at a rate sufficiently rapid to maintain 

 normal respiration. However, if the temperature is raised 

 to 100 F., the rate of respiration is so increased that oxygen 

 cannot penetrate to the inner cells rapidly enough to supply 

 their needs. As a result the internal tissues of the tuber 

 become asphyxiated, die, and turn black. 



If potatoes are kept at normal temperatures but are not 

 well aerated and do not have a normal supply of oxygen, 

 black heart also results. In this case the death and dis- 

 coloration of the cells is not restricted to the heart of the 

 potato. 



Chilling and freezing injury are good examples of the 

 effects of excessively low temperature upon plant tissues, 

 while sunscald is an example of the effect of extremely high 

 temperature. Either extreme kills the tissues. 



Tolerance, Resistance and Predisposition to Disease. 



Many plants have to a certain degree an ability to tolerate 

 unfavorable climatic, soil, and cultural conditions, and to 

 withstand or tolerate the attacks of bacteria and fungi. 

 This tolerance varies with the age and variety of the plant 

 and with the conditions under which the plant is grown. 



Resistance to parasitic attack may be due to the nature 

 and thickness of the protective covering of the plant, to a 

 scarcity of natural openings, to the rapidity with which 

 wounds heal over (with wound cork), to the time of ma- 



