MARKET DISEASES OP VEGETABLES. 7 



DISEASE IN PLANTS. 



Disease in plants is any deviation from the normal con- 

 dition of their functions or tissues. The majority of plant 

 diseases as they occur in the market are localized in their 

 effects and differ therein from most animal diseases, which 

 usually are systemic, and affect the whole body because 

 of the presence of a nervous and a circulatory system. For 

 example, the outer leaves of a celery plant may be rotted 

 severely while the inner leaves or heart are in no way 

 injured and are fit for food. It must be remembered, how- 

 ever, that the sound part of a fruit and vegetable may in 

 some cases acquire a disagreeable odor or flavor from the 

 diseased part. At times even sound fruits or vegetables are 

 affected in taste and flavor by nearby decaying fruits and 

 vegetables. 



Types. 



Diseases may occur in the growing plant, checking and 

 preventing its growth or its production of normal parts or 

 even killing it outright. They may also occur in fruits and 

 vegetables in transit and storage. We may therefore distin- 

 guish between field and transportational diseases, and shall 

 attempt to differentiate as much as possible between dis- 

 eases which originate in the field and those which originate 

 in transit and storage. 



For example, infection of sweet potatoes with soft rot or 

 peaches with brown rot may occur either in the field, in 

 transit, or in storage, whereas infection of sweet potatoes 

 with scurf or peaches with scab occurs only in the field. 

 The first two diseases, however, may develop or progress in 

 the field, in transit, or in storage, whereas the latter two 

 probably do not develop or spread in transit or in storage. 



Symptoms. 



The signs or characteristics of a disease which mark it 

 and by which its presence is discerned, are known as the 

 symptoms of the disease. During the progress of a disease, 

 its symptoms may change. Frequently there are early and 

 advanced symptoms. The early symptoms of watermelon 

 anthracnose, for example, are raised welts on the rind, 

 while the advanced symptoms are deeply sunken lesions 

 with a salmon pink covering of spores. 



Not all the symptoms of a disease necessary for its diag- 

 nosis may appear on one specimen. As a matter of fact, 

 it is often impossible to diagnose a disease by examining a 

 single specimen, and a correct diagnosis often involves an 

 examination of the whole field or the entire lot in the car 

 or in storage. In a shipment of celery, for example, some 

 plants may show a watery rot. This condition alone may be 

 insufficient for a correct and complete diagnosis. Other 

 plants in the same shipment may show a white cottony 



