Transmission of Indian Decline by erytreae (Del Guercio), the Vector of South African Greening

G. Massonie, Monique Garnier, and J.M. Bove

Two ps$lid vectors of greening are Even though Koch's postulates have known. In Africa the disease agent is not yet been fulfilled, it is probably that transmitted by Trioza erytreae (Del the procaryote-like organism. associated Guercio) (McClean and Oberholzer, with all forms of greening and their 1965), while in India, China, and south- psyllid vectors in the casual agent of the east Asia the vector is the oriental psylla, disease (BOV~and Saglio, 1974). (Kuwayama) (Capoor et Since South Africa seems to be in- al., 1967). fested only with the less destructive, Two forms of greening have been char- heat-sensitive form of the greening patho- acterized (Bove' et al., 1974). One is heat gen the introduction of the more virulent, sensitive (no symptoms above 27 to Asian form would be detrimental to the 32" C); it is the form present in South South African citrus industry, especially Africa. The other form is heat tolerant if the African vector, T. erytreae, the (good symptoms at 30 to 3S°C) and is only one known to be present in South prevalent in India (citrus decline), Taiwan Africa, should prove capable of trans- (likubin), the Philippines (leaf mottling), mitting the disease. The work reported and China (yellow shoot). The Asian, here shows that T. erytreae can indeed heat-tolerant form of the disease is much transmit the destructive, heat-tolerant more destructive than the heat-sensitive, Indian greening. African form.

MATERIALS AND METHODS The Indian greening used in this work procedure was repeated twice. Three was from Poona and has been described psylla generations were thus obtained. earlier (BovC et al., 1974); it is free of any The adults from the third generation were other known pathogen. Young adults of used for acquisition feeding on a Hamlin the African citrus psylla, T. erytreae, col- sweet orange seedling with Indian lected near Pretoria on citrus trees show- (Poona) greening. This seedling showed ing no symptoms of greening were carried severe symptoms of the disease but still to Bordeaux, France by Dr. D. Catling. had young flushes of growth, The adult On December 30, 1971 they were placed psyllas from the third generation were in -proof cages on healthy young left on the diseased Hamlin seedling until Hamlin sweet orange seedlings. These eggs had been laid and were then removed cages were kept in a large growth cham- and discarded. About 100 adults (fourth ber held at 24O2 loC during the 16-hour generation) were produced; they were light period (1 5,000 lux) and at 21 ?r lo C divided among 5 healthy sweet orange in the dark (8 hours). Relative humidity seedlings and each seedling was put in an was 80 2 10 per cent. individual cage. Transmission feeding was After one month a new generation of from April 20 to May 9, 1972. psyllas was obtained and the young adults Electron microscopy techniques are were again transferred to healthy seed- described elsewhere (Garnier et al., lings under the same conditions. This 1976). Stubborn, Greening, and Related Diseases 19

Fig. 1. (A) Electron micrograph of a thin section through the phloem of a Hamlin sweet orange seedling showing symptoms of greening 10 months after transmis- sion feeding by Trioza erytreae; bodies present in the sieve tube represent the procaryote-like structures associated with greening and are probably the etio- logical agent; (B) enlargement of the upper part of A. Seuenth IOCVConference

RESULTS AND CONCLUSION The Hamlin sweet orange seedlings on symptoms. which the three generations of psyllas It is known that the phloem tissue were reared were kept, after the from greening-affected citrus contains had been removed, in a greenhouse procaryote-like "structures," surrounded between 20 and 27"C, a temperature by a 25 nm-thick envelope system, which range adequate to allow symptom expres- probably are the etiological agent of the sion of African greening (Bod et al., disease (Bove' and Saglio, 1974; Garnier et 1974). During the following 2 years none al., 1976). Figure 1A and B (next page) of these seedlings showed symptoms of shows these "structures" in the sieve tubes greening; they were then discarded. It can of the one Hamlin seedling which showed thus be safely assumed that the psyllas greening symptoms. used for the acquisition feeding were free It is thus clear that this seedling was of the greening pathogen. infected with the heat-tolerant Indian The five Hamlin seedlings used for form of the greening pathogen, because transmission feeding were placed in a severe symptoms developed at 27 to growth chamber kept at 27 -+ 2" C in the 32" C, at which temperatures the heat- dark (8 hours) and 32 k 2" C in the light sensitive South African form of the green- (16 hours at 20,000 lux). Thls temper- ing pathogen does not cause symptoms ature range favors symptom expression of (Bovt? et al., 1974). heat-tolerant strains of greening to which These experiments demonstrate that the Poona, Indian form belongs. After 10 T. erytreae is able to transmit a heat- months, one of the five Hamlin seedlings tolerant strain of the greening pathogen began to show typical symptoms of and that the introduction into South greening: leaf mottling, zinc deficiency Africa of such a strain could represent a patterns, young twig dieback, etc. None serious threat to the citrus industry. of the other four seedlings ever showed

LITERATURE CITED

BOVE, J. M., E. C. CALAVAN, S. P. CAPOOR, and R. E. SCHWARZ 1974. Influence of temperature on symptoms of California stubborn, South Africa greening, India citrus decline and Philippines leaf mottling diseases, p. 12-15. In L. G. Weathers and M. Cohen, (eds.), Proc. 6th Conf. Intern. Organization Citrus Virol. Univ. California Div. Agr. Sciences, Berkeley. BOVE, J. M., and P. SAGLIO 1974. Stubborn and greening: a review, 1969-1972, p. 1-11. In L. G. Weathers and M. Cohen, (eds.), Proc. 6th Conf. Intern. Organization Citrus Virol. Univ. California Div. Agr. Sci- ences, Berkeley. CAPOOR, S. P., D. G. RAO, and S. M. VISWANATH 1967. Diaphorina cirri Kuway, a vector of the greening disease of citrus in India. Indian J. Agr. Sci. 37:572-76. / GARNIER, M., J. LATRILLE, and J. M. BOVE 1976. Sprioplasma citri and the organism associated with likubin: comparison of their envelope systems. (This volume). McCLEAN, A. P. D., and P. C. J. OBERHOLZER 1965. Citrus psylla, a vector of the greening disease of sweet orange. S. African J. Agr. Sci. 8:297-98.