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1916 ANNUAL REPORT 



A BARK DISEASE OF AVOCADO TREES 

 By Howard S. Fawcett, Pathologist, Citrus Experiment Station, 

 Riverside, Cal. 



Description of the Disease. — In May, 1914, attention was called to a 

 bark disease of avocado trees by K. A. Ryerson, who sent part of a dis- 

 eased trunk of an avocado tree with the request for information regarding 

 the nature of the trouble. The bark was killed and slightly sunken over 

 an area of 8 inches in length and 2^4 inches (one-half the circumference) 

 in greatest width. A kind of gum had exuded and hardened in small beads 

 on the surface, and in addition a white, powdery, chrystalline substance 

 was deposited over the surface at the lower part and below the diseased 

 area. The freshly killed bark and wood were still firm, not soft or watery. 

 The wood was not affected more than one-eights of an inch inward. This was 

 typical, as was afterwards learned, of a bark disease occasionally occur- 

 ring on avocados in a number of localities in Southern California. The 

 following information regarding its occurrence was obtained from Mr. 

 Ryerson. The disease occurs most often on the trunk not far from the sur- 

 face of the soil, but occasionally it is found higher up and on limbs, begin- 

 ning especially at the base of a leaf. It occurs not only on the trunks of 

 larger trees, but is found on small seedlings, especially if overwatered or 

 if the drainage is not good. This disease, while not frequent enough to 

 cause serious damage unless on particularly valuable trees, is likely to 

 appear suddenly and spread fairly rapidly in spite of care exercised to 

 check it. It does not appear to be confined to any particular variety, but 

 has been found on a number of different kinds. 



Isolation of Pythiacystis Fungus. — Three culture tests were made 

 from this specimen from Mr. Ryerson, by flaming the surface, cutting 

 off the surface of the bark, flaming again, cutting out small bits of the 

 bark and wood from the junction of the dead and live tissue, and drop- 

 ping them into slant cornmeal agar tubes. A Pythiacystis fungus grew 

 out from the pieces in the three tubes. 



In July, 1914, specimen of an affected avocado limb which had been 

 unsuccessfully treated by cutting away the diseased bark and disinfecting 

 the area, was sent from the same locality as the first. The bark had be- 

 gun to heal at the cut edges, but the wood underneath was dark in color. 

 The same fungus was again isolated from three different places in the 

 darkened wood about one-fourth of an inch from the surface and in one 

 case 4 inches beyond the point where bark had been cut away for treat- 

 ment. In five out of six tubes made as before, the Pythiacystis fungus 

 grew out from the pieces. 



Inoculation Experiments With the Fungus. — On October 6, 1914, the 

 trunks of two Mexican seedling avocado trees at the Whittier laboratory 

 were inoculated with this Pythiacystis by inserting into longitudinal cuts 

 three-fourths of an inch long, bits of the mycelium from the culture of 

 the fungus. The cuts were then covered with paraffined paper held with 



