CLITOCYBE ROOT ROT OF WOODY PLANTS 19 



The disease has been found to develop on land formerly occupied by 

 native hardwood forests, attracting no particular attention until from 

 a few to several years after the land is cleared and planted to orchard 

 or ornamental trees. The Clitocybe root rot fungus may remain viable 

 for several years in infected roots left in the land at the time of clear- 

 ing. These serve as sources of infection to trees subsequently planted. 

 Thus trees, even in orchards and other types of plantings, may contract 

 the disease after several years in spots where the existence of the 

 causal fungus was not even suspected. Occasional instances of 

 Clitocybe root rot have been observed in residential properties where 

 the land had been cleared for 20 or more years. In some cases trees 

 have grown from 20 to 35 or 40 years before succumbing to root rot. 

 The occurrence of the disease in such cases does not appear to be 

 explainable on the basis of the roots coming into contact with old 

 infected roots left in the soil at the time the land was cleared. It can 

 only be inferred that infection may at times develop in some way other 

 than by root transmission, presumably from spores. 



The incidence of Clitocybe root rot in situations in Florida where 

 hardwood timber occurs, or was prevalent prior to clearing, has been 

 found to be by far the greatest on well-drained, light, sandy soils, 

 which are dominantly acid in reaction and commonly subject to 

 drought at frequent and often for prolonged periods. The disease 

 also occurs in western Florida and Alabama on clay soils that are also 

 acid. Losses from Clitocybe root rot occur infrequently on low ham- 

 mock soils in Florida. Although these soils are characterized by the 

 prevalence of oak and other hardwood trees prior to clearing, they 

 are heavier and of greater moisture content than sandy soils, and 

 usually neutral or alkaline in reaction, often being underlaid closely 

 by marl or shell. On the other hand, the disease is unknown in Dade 

 County, Fla., even in the extensive plantings of Australian pines and 

 areas of natural reproduction of the horsetail Australian pine from 

 Miami to Homestead. While oak trees occur here, the soils of this 

 county are almost invariably alkaline, being closely underlaid by 

 oolitic limestone. In view of this and of the fact that the disease 

 sometimes occurs on land that has been cleared for many years and 

 is free from old roots, it appears that some factor or factors other 

 than the occurrence of infected roots in the ground to serve as sources 

 of infection are involved in the incidence of the disease. 



Clitocybe root rot also appears to be absent in the typical flatwoods 

 sections of Florida, where the dominant vegetation consists of pine 

 trees and saw palmetto, and oak and other hardwood trees do not 

 occur. It also appears to be absent in prairie lands. Coastal plant- 

 ings of the extremely susceptible horsetail Australian pines in south- 

 ern Florida on land made by dredging sand and shell from under 

 salt water also appear to be characterized by complete freedom from 

 root rot. 



CONTROL MEASURES AND TREATMENT OF DISEASED 



TREES 



Clitocybe root rot is one of those diseases against which it is much 

 easier to take precautions in advance of planting than it is to apply 

 remedial measures after the disease develops. Experience demon- 

 strates that plantings of susceptible woody plant species on land where 



