3.7.11
GUAVA WILT DISEASE -- SELECTION FOR RESISTANCE

MH SCHOEMAN and JE VOS

ARC- Institute for Tropical and Subtropical Crops, Private Bag X11208, Nelspruit, 1200, South Africa

Background and objectives
Wilt of guava (Psidium guajava) caused by the fungus tentatively identified as Penicillium vermoesenii, has had a devastating effect on the South African guava industry which is based on the highly susceptible Fan Retief cultivar. As a result of guava wilt disease (GWD) the total area under guava trees in the Southern Lowveld area of the Mpumalanga Province of South Africa decreased by 80% from 1982 to 1994. Symptoms include wilting, chlorosis and defoliation [1]. No chemical control measures are effective, consequently a cultivar with an adequate degree of resistance is required. Three promising selections have been selected using a tissue culture selection programme [2]. In this study these selections, known as TS-G1, TS-G2 and TS-G3 were evaluated for resistance or tolerance to GWD in glasshouse tests and in the field.

Materials and methods
Non-grafted selections were inoculated with the fungus in the stems or in the roots. In another trial, resistance of the three selections was evaluated by grafting Fan Retief on each of these selections as scions and subsequently inoculating the roots. In both trials Fan Retief was used as the control. In a field trial resistance of these selections was evaluated by identifying 15 sites in an existing Fan Retief orchard where trees have died of GWD and replacing the dead trees in each of the 15 sites with one tree of each of the selections as well as a Fan Retief tree.

Results and conclusions
Six months after inoculation, all the Fan Retief plants in the non-grafted experiment inoculated in the stem or in the roots, were dead. Except for one plant of selection TS-G2, inoculated in the stem, none of the plants of the other selections showed any symptoms. In the grafted trial, 100% of the Fan Retief plants grafted onto Fan Retief were dead six months after inoculation, while only one plant grafted onto selection TS-G3 showed symptoms. In the field trial three Fan Retief plants were dead three years after planting while none of the plants of the other selections showed any symptoms. These results indicate that these selections are more resistant to GWD than the commercial Fan Retief cultivar. Selection TS-G3 appears to be tolerant to GWD since the Fan Retief scion was affected by the fungus. Since February 1996 selections TS-G1 and TS-G2 have been used as rootstocks for Fan Retief in commercial plantings in South Africa.

References
1. Schoeman MH, Benade E, Wingfield MJ, 1997. Journal of Plant Pathology 145, 37-41.
2. Vos JE, Schoeman MH, 1994. Abstracts V111 th International Congress of Plant Tissue and Cell Culture, Firenze, Italy, June 12-17, 1994.