THE MONTHLY BULLETIN 



CALIFORNIA STATE COMMISSION OF HORTICULTURE 

 \'ol. IV. July, 1915. No. 7. 



MELAXUMA OF THE ENGLISH WALNUT.* 



By Howard S. Fawcett, Associate Professor of Plant Patholog>% Citrus Experiment 



Station, University of California. 



Melaxuma, a disease causing black cankers and the exuding of "black 

 sap ' ' on the large limbs and trunks of English walnut trees, has become 

 of considerable importance in Santa Barbara Count}- during the past 

 two or three years. The disease has been found only to a small extent 

 in Ventura, Los Angeles and Orange counties, and does not seem to be 

 nearly so serious in its attack on trees in these counties. InvestigatiQus 

 covering two years liave shown that the disease is infectious, that the 

 causal agent is a fungus and that the cankers may be checked by cutting 

 out and disinfecting the wounds. Although the word Melaxuma means 

 "black sap," there are frequent cases of black sap oozing from walnut 

 trees without the presence of this disease. Black sap may ootze out 

 after large limbs have been cut off, or may sometimes be seen oozing 

 out from injuries made in cultivating an orchard, or following sunburn 

 or frost injuries. This may be quite independent of the canker disease, 

 Melaxuma. 



In June, 1913, while the writer was with the State Commission of 

 Horticulture, Sacramento, an investigation of this Melaxuma or canker 

 disease was begun at the request of Mr. C. W. Beers, Horticultural 

 Commissioner of Santa Barbara County. The disease was found to 

 be characterized by definite areas of killed bark on the trunk and large 

 limbs, and in severe cases also further out on the smaller branches. 

 Young trees appear to be rarely affected. The cankers found on the 

 larger limbs are characterized usually by sunken areas from a few 

 inches to even a foot or more in length. Both the bark and the wood 

 underneath become black, and during the actively growing period of 

 the tree a considerable amount of the black sap will form just under- 

 neath the bark. As the disease progresses the bark shrinks, cracks and 

 this black sap runs down and stains the limbs below the cankers, 

 (Fig. 62). The disease may start on a small-sized twig and after 

 killing it spread in an oval area to the bark of the branch bearing this 

 twig. In severe attacks smaller twigs are rapidly girdled, and the 

 whole branch beyond the affected place suddenly withers and the dead 

 leaves remain attached to the branch. The most common place for 

 the cankers to start is in injuries made by carelessness in cultivating 

 the orchard, or at the crotches w^iere the first large branches start out. 

 In the development of these cankers, after the bark first cracks and 

 the black sap oozes out, there is a tendency for new tissue to form 

 around the edges and retard the further spread of the diseased area. A 

 few of the cankers may heal up entirely, but this is rare. A canker 



*Paper No. 15, Citrus Experiment Station, College of Agriculture, University of 

 California. Riverside, California. 

 17975 



