Reprint From CMS Yearbook 1973

There are relatively few diseases affecting the Macadamia tree, so that this nut crop has a minimum of disease control procedures with which to contend. Diseases, which have to date occurred only sporadically, include Phytophthora trunk canker (1, 7, and 9), Phytophthora blight flower (4), Botrytis blossom blight (2, 3, 5), Dothiorella branch canker (10) and crown gall (7).

In California the disease situation has remained relatively unchanged in recent years. Our first report of Phytophthora canker was in 1960 (8, 9), and very few additional cases have been seen since the original disease occurrence in Vista. Plantings of Macadamia trees have not been extensive in California, but a sizeable number of trees have been replanted in avocado root rot areas, with very little disease occurrence. It appears that Macadamia roots are resistant to Phytophthora and that primarily the trunks are invaded if there is a wound.

More cases of Phytophthora trunk canker have appeared in Hawaii, primarily on the island of Hawaii (1), than in California. Recently there has been a considerable increase in occurrence of Phytophthora canker in Queensland, Australia (6), in some of the new plantings of Macadamia in several areas in that part of Australia.

There are two mating or compatibility types of the fungus Phytophthora cinnamomi A1 and A2. My first isolates of the fungus from Hawaii were the A1 type; this was the first report of that type for the fungus. We have isolated both types from Macadamia cankers in Hawaii, and the A2 type from cankers in California.

Our inoculations at the time that the disease was first described and proven to be caused by P. cinnamomi showed that Macadamia integrifolia is more susceptible to Phytophthora than M. tetraphylla. The report of disease incidence from Australia is mostly with M. integrifolia. Dr. W. H. Storey has recently provided me with seeds of another species of Macadamia, M. ternifolia, and inoculations of seedlings of this species will soon be made.

Little research has been done on the other diseases of Macadamia. Dr. J. E. Hunter and coworkers at the University of Hawaii Experiment Station at Hilo reported in 1971 that Phytophthora palmivora and P. nicotianae var. parasitica can cause drastic reduction of fruit set by attack on the flower (4). These two species of Phytophthora destroy many of the racemes and nuts under favorable conditions for development of the fungi, which include cool temperature and prolonged rainy periods, on the island of Hawaii, P. nicotianae var. parasitica survives in dead racemes remaining attached to the branches, and thus serves as a source of inoculum for infections in the following flowering season. P. Palmivora does not survive in dead racemes.

The fungus Dothiorella (Botryosphaeria ribis) is occasionally a problem on many different crops, including avocado (IC), berries, apples, and other deciduous fruit trees. On Macadamia it occurs only occasionally and can cause cankers on branches, which result in dying back of the branches.

Recent experiments in Hawaii have demonstrated that the fungus Botrytis cinerea can cause significant reduction of nut set by attacking the flowers under the high rainfall conditions prevailing on the island of Hawaii (2, 3, 5). Sprays with the systemic fungicide benomyl provide good protection and consequent significantly high increases in nut set. Another disease reported from Hawaii is crown gall, caused by the bacterium Agrobacterium tumefaciens (7). This apparently is a very minor problem, with one case of the disease reported on a Macadamia seedling on the island of Hawaii. No additional reports of this disease have appeared since the original description by Dr. Raabe in 1964.