16 CIRCULAR 853, U. S. DEPARTMENT OF AGRICULTURE 



Mechanical injury is not necessary for infection, since it has been 

 observed repeatedly in field investigations that, like Armillaria mellea, 

 O. tabescens can penetrate healthy roots readily through uninjured, 

 bark. The parasitic nature of this fungus is apparent from the readi- 

 ness with which it attacks the most healthy and vigorous plants and 

 the ease with which it may spread from one tree to another through 

 root contact. 



RELATIVE SUSCEPTIBILITY AND RESISTANCE OF HOST 

 PLANTS IN FLORIDA 



Little information is available on the comparative susceptibility and 

 resistance of various plants to Clitocybe root rot, even in Florida 

 where the disease has been studied rather intensively. It is difficult 

 to gather such data on the basis of the occasional dying of miscel- 

 laneous plants on various properties, since one rarely finds sufficient 

 plants on individual properties to make reliable comparisons. Under 

 these circumstances, comparisons of the susceptibility and resistance 

 of plants can be made only by considering them with respect to the 

 general prevalence of the disease in a large number of plantings in 

 various parts of the State. It must be borne in mind, however, that 

 the likelihood of susceptible plants being attacked varies greatly, ac- 

 cording to the site on which they are planted. For example, even 

 the extremely susceptible Australian pines may prove fairly free 

 from root rot when planted on low hammock, prairie, flatwoods, coastal 

 beach, or other land that is free from infection, but very subject to 

 it when planted on well-drained, sandy land where oak trees grew 

 prior to clearing or some oak scrub remains. 



The repeated occurrence of Clitocybe root rot on various species 

 of Australian pine in widely scattered localities in Florida clearly 

 demonstrates that these trees head the list of susceptible plants. These 

 exotics are utilized extensively in Florida as windbreaks for citrus 

 and other subtropical fruits, street and roadside trees, and screen, 

 hedge, and other ornamental plantings. Aside from the susceptibility 

 of some species to cold, their extreme susceptibility to this disease 

 has proved the most serious drawback in many parts of the State to 

 the extensive plantings of these very useful and extremely fast- 

 growing trees. The inroads of Clitocybe root rot frequently result 

 in such widespread mortality of Australian pines that their usefulness 

 or esthetic value is often rapidly impaired, or even virtually destroyed, 

 within a few years. 



Peach trees also are extremely susceptible to Clitocybe root rot. 

 This disease has proved to constitute an important factor of loss in 

 a number of commercial plantings of tung trees. Citrus trees on 

 rough lemon stock likewise have proved quite susceptible to Clitocybe 

 root rot, although it has not been found in those on sour orange stock. 

 The common guava, sturdy cypress-pine, turkscap waxmallow, Amer- 

 ican arborvitae, eucalyptus, azaleas, and other plants have proved 

 very susceptible. The disease also has been found to be the major 

 cause of the dying of roses in Florida. 



A general idea of the relative frequency of occurrence of Clitocybe 

 root rot on the more commonly cultivated plants in Florida is given 

 in table 1. The frequency with which it has been recorded on these 



