POMONA COLLEGE JOURNAL 



of ECONOMIC BOTANY 



Volume I SEPTEMBER 1911 Number 3 



The Withertip Disease in Florida 



P. H. ROLFS 



DIRECTOR OF THE FLORIDA AGRICULTURAL EXPERIMENT STATION 



In the article on Withertip in the first number of the Pomona College Journal 

 of Economic Botany there has been brought together a very complete and 

 forceful resume of this subject as we now know it. Our laboratory experiments 

 with pure cultures show conclusively that withertip fungus is able to infect healthy, 

 uninjured tissue not only of oranges but of a number of other sub-tropical fruit 

 trees. Naturally the disease is more serious on trees whose constitution has been 

 weakened by other untoward conditions. Consequently, the average grower, 

 whose opportunities for observation and close study are more limited than those of 

 the plant pathologist, is entirely justified in coming to the conclusion that the 

 withertip fungus is a secondary parasite. 



In regard to the red spots of an anthracnose-like character which frequently 

 occur on citrus fruits, I may say that mistakes are frequently made. We know 

 that the resultant lesions from certain chemical poisons on fruit are so like those 

 produced by the withertip fungus that they are not easily distinguished even by 

 use of a compound microscope. In pronouncing a case anthracnose, therefore, the 

 experienced plant pathologist does so with a great deal of hesitation unless he is 

 very familiar with the surrounding conditions. For instance, I should hesitate 

 very much in pronouncing a lesion due to withertip by examining a specimen of 

 fruit only, due to the fact, which I have described above, that chemicals are known 

 to produce lesions that are very similar to the withertip lesions on mature and 

 nearly mature fruit. In the groves, however, and where all the surrounding con- 

 ditions are known to the plant pathologist, he should be able to make a diagnosis 

 without much hesitation. 



I was very much surprised during my visit in California by the frequency 

 with which I met this fungus. In most of the cases it was simply hanging around 

 in the groves and doing very little damage, and no one but a person who was 

 familiar with the fungus at first-hand would have suspected its presence. 



In our early studies of the withertip fungus we laid a great deal of stress on 

 the use of Bordeaux mixture. Our later and more exact work has enabled us to 

 treat the disease very successfully without the use of this spray. Everybody who 

 has had experience with Bordeaux mixture on a large scale knows that it is a rery 

 disagreeable material to handle. In Florida, also, our friendly fungi hold the 

 scale insects so much in check that we hardly need to spray for them. By using 



