CLITOCYBE ROOT ROT OF WOODY PLANTS 13 



CAUSAL FUNGUS 



Clitocybe root rot is caused by Clitocybe tdbescens (Fr.) Bres., one 

 of the gill fungi. The fruiting bodies of this fungus consist of a few 

 to many individuals, with the stems developing from a common base 

 (fig. 9, A) . When fully developed, the caps are convex to flattened or 

 centrally depressed with age, whitish to light tan or honey-colored, 

 smooth, or adorned with tufts of fibrils near the center, and from 2 

 to Sy 2 inches in diameter, with whitish gills underneath (fig. 9, B). 

 The united Clitocybe, as this fungus is commonly called, resembles the 

 closely related honey agaric (Armillaria mellea) in habit of growth, 

 color, texture, and general appearance, but is distinguished principally 

 by the absence of an annulus on the stem, dissimilar spores, and in 

 being more slender from the beginning. It is considered superior to 

 A. mellea in flavor and edibility. The latter fungus is of cosmopolitan 

 occurrence and causes a very similar root rot. 



The production of fruiting bodies by the Clitocybe root rot fungus 

 depends largely upon the progress of the disease and the seasonal 

 conditions. They develop with greatest frequency during the fall, 

 though in Florida they occur most commonly from the middle of Sep- 

 tember to the middle of October, but may develop also in other months 

 if favorable soil moisture conditions prevail. As a rule, only one or 

 two clusters develop during the year and frequently none develop. 

 They usually attain their full development within a few days from 

 the time the young "buttons" appear and decay rapidly in warm, rainy 

 weather. However, if dry weather follows their development, the 

 clusters may dry up and turn dark brown to blackish but remain 

 recognizable for several weeks to one familiar with the fungus. The 

 presence of either fresh or old clusters of the fruiting bodies of the 

 fungus frequently makes it possible to locate diseased trees, especially 

 citrus, even before the tops begin to decline or the presence of the 

 disease is suspected. 



It should be borne in mind, however, that by no means every mush- 

 roomlike or toadstoollike fungus that may occur occasionally at the 

 bases of trees is the particular species causing Clitocybe root rot. 

 Aside from differences in character, other fungi are unlikely to be 

 growing in clusters with several individual fruiting bodies arising 

 from a common base at the foot of the tree and certainly will lack 

 organic connection with the bark of the trunk or roots. 



ISOLATIONS OF THE FUNGUS 



Clitocybe tdbescens has been isolated consistently from a diverse 

 array of trees, shrubs, and vines attacked by mushroom root rot in 

 various parts of Florida from 1924 to 1944. The only other organisms 

 developing in these isolations were the ubiquitous bacteria, molds, and 

 soil-inhabiting fungi that may be expected to occur as contamina- 

 tions. In most cases a large percentage of pure cultures of C. tabescens 

 resulted, while in others a large percentage of contaminations oc- 

 curred. The root rot fungus develops very slowly, usually requiring 

 from 1 to 2 weeks, and occasionally longer, for growth to begin. As 

 a result, if any contaminations are present, they usually develop to 

 the exclusion of the desired fungus. 



