P. C. J. Oberholzer, I. Mathews, and S. F. Stimie investigated the decline of grapefruit trees in South Africa and concluded that stem pitting of grapefruit was readily perpetuated by budding and was probably of virus nature. Certain aspects of the disease symptoms bore considerable resemblance to blind pocket psorosis, and they suggested that the disease is caused by a strain or strains of the psorosis virus. Their investigations indicated that the disease was confined largely to the grapefruit scion and that the rootstock did not seem to be of importance, although they thought the latter should be investigated further.

The report of their studies stimulated a series of observations and studies in several parts of the world. A. S. Costa, T. J. Grant, and Sylvio Moreira, after study of their tristeza-inoculated plant material in Brazil, suggested a possible relationship between tristeza and the stem pitting disease of grapefruit in Africa. A. P. D. McClean, of South Africa, pointed out similarities of the stem pitting disease of grapefruit and a disease of lime plants in the Gold Coast. Subsequently he presented further evidence of the similarity of the tristeza disease, the stem pitting disease of grapefruit, and the lime disease. In 1951 J. M. Wallace and R. J. Drake, of California, reported stem pitting of West Indian lime plants affected with quick decline. Thus, again, the combination of cooperative and independent investigation in many parts of the world brings us one step closer to an understanding of the great similarity of these diseases and the varying effects that they have on different citrus types and varieties.

The recognition of the similarity of these diseases is important as it indicates that in the case of grapefruits and limes the use of tolerant rootstocks does not furnish an adequate means of insuring the economic production of these under tristeza disease conditions. The knowledge, however, that there are different strains of the causal virus and the indication from the work in Brazil that infection with milder virus strains protects plants from the severe forms of disease may eventually be employed to produce grapefruit on tolerant rootstocks even in tristeza-affected areas.

The oriental black citrus aphid is recognized in South America and in South Africa as the insect vector of the virus. That aphid is not known to exist in the United States. The results of studies by R. C. Dickson, R. A. Flock, and M. McD. Johnson at the California Citrus Experiment Station indicate that the melon aphid is a vector of the quick decline virus, but apparently is not a particularly efficient carrier of the virus because only a small number of transmissions were obtained in a large number of tests. Prevalence and efficiency of the vectors are major factors in the spread of the disease under field conditions and of interest to all citrus growers, especially in newly affected areas. Much remains to be learned concerning insect vectors and their ability to transmit the mild and severe forms of the causal virus of the disease. In all new areas where the disease is found it is essential to determine the severity of the virus strains present and to test for the presence of insect vectors.

T. J. Grant returned from Brazil in 1951 and undertook the work of testing for presence of the tristeza virus in the citrus areas of Florida. By November of that year the first series of seedlings of West Indian lime test plants were inoculated by means of bottle grafts. Twigs were used from field trees in a state of decline. In February of 1952 the first vein-clearing symptoms on the lime test plants were observed. Subsequently bark samples from other declining field trees were examined by Henry Schneider. His results, plus those from inoculated test plants, showed that a strain of the tristeza, or quick decline, virus was present in Florida. This proof was followed by action of the Florida State Plant Board and Citrus Experiment Station and a system for orchard inspection and testing of suspicious cases was established.

The reaction of the infected lime test Plants held under controlled conditions suggests that the strain of the tristeza virus present in Florida is mild compared to the severe virus strain present in Brazil. The individual and scattered groups of infected trees in the Florida orchards suggest that the insect vector is not an efficient carrier of the virus. It is estimated that the mild strain of tristeza virus has been in Florida since 1942 or longer. The reactions of the lime test plants also indicate that frequently under field conditions declining trees are carrying more than just the tristeza virus. It seems likely that with intensive research investigations there eventually will be discovered several new virus diseases of citrus.

After the virus nature of the tristeza and quick decline diseases was recognized, it was soon learned in Brazil and California that the sweet orange tops on sweet orange rootstocks could be carriers of the virus even though they showed no symptoms.

If trees of certain rootstock-scion combinations could carry the causal agent of the disease without showing any visible symptoms, the disease might be carried from affected to disease-free areas. Quarantine officials would have no way of knowing that the apparently healthy plant material harbored a virus that could cause great losses to the citrus industry. This is equally true of the grower who in visiting an orchard and observing a new variety cuts off some budwood to carry home. In so doing he may very well be the means by which the virus is brought into a disease-free area. E. P. DuCharme and L. C. Knorr, sent to Argentina by organizations of citrus growers in Florida and Texas and in cooperation with the Florida State Citrus Experiment Station, reported that the disease very likely reached Argentina between 1927 and 1930 on two large shipments of Rough lemon rooted nursery stock from South Africa.

Nurserymen have no quick test to distinguish between the healthy plants and apparently healthy ones that carry the virus. Eradication of the disease therefore would be difficult. The simple method of removing or destroying only the trees that showed symptoms would be completely inadequate as many of the remaining trees could be symptomless carriers of the causal virus. The formulation of quarantine measures is primarily a local problem, which will have to be given specific attention in each newly affected area.

Although the recommendations as to what to do about the disease in areas already affected must be determined by local conditions, no rootstock found to be nontolerant of the disease should be employed in such areas. The tolerant rootstocks most desirable for a specific region are sometimes known but for horticultural reasons are not necessarily satisfactory in other regions. Hence there is need for rootstock test plantings but in any such test plantings there is a predominant need to be absolutely sure that the budwood employed is free of all transmissible diseases.

Strains of the tristeza, or quick decline, virus are known to be present in citrus orchards in California, Louisiana, and Florida. The interaction of these with other citrus diseases has yet to be investigated thoroughly. That is necessary for the establishment of a sound basis for the continuation of profitable production of citrus; it is especially necessary in the case of grapefruit, in which even the mild tristeza virus strains alone can cause some damage. The first important step in the reduction of losses occasioned by bud-transmissible disease is the establishment of effective methods for bud certification. California and Texas have bud-certification programs, and the organization of one was started in Florida.

T. J. GRANT is a member of the Bureau of Plant Industry, Soils, and Agricultural Engineering stationed in Orlando, Fla.

L. J. KLOTZ is head of the division of plant pathology in the University of California, at Riverside.

J. M. WALLACE is a plant pathologist in the Citrus Experiment Station of the University of California.