18 CIRCULAR 853, U. S. DEPARTMENT OF AGRICULTURE 



found to be attacked occasionally do not appear to be particularly 

 susceptible. It has been noted that the occasionally infected oleanders 

 and silk-oak Grevilleas sometimes fail to develop the disease even 

 though they occur in, or closely adjoining, plantings where Clitocybe 

 root rot has killed Australian pine trees over a period of years. 

 Moreover. Chinese hibiscus appears to be much more resistant than 

 turkscap waxmallow when they are planted together in situations 

 where root rot is active. Palm trees have rarely been found attacked 

 and appear to be quite resistant. The writer observed a striking 

 example of the way coconut and royal palms completely withstood 

 the Clitocybe root rot fungus in a mixed planting in which the ma- 

 jority of the horsetail Australian pines alternating with these palms 

 died at an early age. 



Clitocybe root rot has not been found thus far on allamandas or 

 crotons, which occur commonly in many ornamental plantings where 

 other plants have died from root rot. The mango has not been found 

 attacked, though occasionally growing in or near plantings where Aus- 

 tralian pines have died, and only two instances of root rot have been 

 observed on avocados. No instance has been found as yet on the 

 widely planted cabbage palmetto. Citrus trees on sour orange root- 

 stock have proved immune thus far, even in groves in which Australian 

 pines planted close to or among them have died extensively. TThile 

 it is premature to state whether these or other trees will prove im- 

 mune, the evidence available indicates that many plants clearly are 

 much more resistant to the disease than others and that at least a few 

 appear to be highly resistant, if not practically immune. 



TThile a considerable number of trees native to Florida have been 

 found attacked, most of these have been attacked but infrequently. 

 The failure to find native forest trees attacked more frequently than 

 has been recorded may be due in some measure to relatively little 

 study having been made of the occurrence of this disease in forested 

 areas. While the oaks are favorite hosts for the saprophytic develop- 

 ment of the fungus, living trees do not appear particularly susceptible 

 to infection, especially when young. Although fairly mature oak 

 trees in woods, partially cleared and developed areas, or yards of resi- 

 dential properties occasionally develop Clitocybe root rot, it has been 

 observed that the progress of this disease is often slow, with several 

 years being required to kill the trees. Judging by the resistance of 

 oaks and other native trees frequently found growing in situations 

 where large numbers of Australian pine and other exotic trees have 

 succumbed to root rot at an early age, the native woody plants of Flor- 

 ida appear vastly more resistant than many of those introduced from 

 foreign countries or other States. 



RELATION OF INCIDENCE OF THE DISEASE TO TIMBERED 

 LANDS AND SOIL CONDITIONS 



Investigations on Clitocybe root rot in a number of States, notably 

 Oklahoma, Missouri. Arkansas, Louisiana, Mississippi, and Florida, 

 have shown that the distribution of this disease coincides to a large 

 extent with the location of area- where hardwood timber, particularly 

 oak trees, occur or were prevalent prior to clearing. As a general rule, 

 it is absent on prairie and other lands where trees, particularly oaks, 

 do not occur, and also on land where pure stands of pine timber occur. 



