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CROWN GALL ON ROSES IN ZIMBABWE SOMETHING OF THE PAST
by Fienie Niederwieser
Agricultural Research Commission, South Africa
The 1600 ha cut rose industry in Zimbabwe was recently threatened by a bacterial disease called crown gall. The disease cannot be cured by any known chemical and the only way to manage the disease is to destroy diseased plants and then
to start fresh with disease-free plants. To solve the problem, a plant propagation scheme was designed and 20 000 disease-free plants were imported from the South African Agricultural Research Council.
The production and export of roses in Zimbabwe is a remarkable success story for Africa in the international floriculture industry. The first production areas were established in 1984 and has grown to more than 1600 ha in 17 years. As a result of the exponential growth rate in the late 1980's and 90's, hundreds of thousands of rose plants were imported from countries such as South Africa, France and Israel. As the industry became firmly entrenched, a number of propagators became established. Currently, most propagation of rose plants takes place in Zimbabwe and little is imported.
During the late 90's, the rose industry was faced with a growing problem of a disease that infected large numbers of production plants, namely crown gall. Crown gall can not be cured by any known chemical. The only way to manage the disease is to destroy infected plants and start fresh with disease-free material. What alarmed propagators and producers the most, was the fact that most of
the rootstock plants were infected with the pathogen and that there was no reliable source of uninfected plants available in Zimbabwe.
Crown gall is caused by the bacterium Agrobacterium tumefaciens. Symptoms are galls on various parts of rose plants, but most often in the crown, hence the name crown gall. The disease causes a reduction in yield and is transmitted through infected cuttings, shears and other implements, as well as infected soil and water. Disease development is favoured by warm temperature but infected plants do not always develop symptoms. Identification of the pathogen
is symptomless, and identification in infected plants is difficult. Plants need to be indexed for two years before one can assume that a plant is free of the disease. One can thus spread the disease unknowingly through propagation material. This probably explains why the disease is less of a problem in countries with cold winters. Crown gall is an important disease in South Africa on grape vines and fruit trees and a lot of research has been done over the years on the disease.
Cutflower producers in Zimbabwe make use of plants on rootstock and most production takes place in soil. The most popular rootstock is Rosa multiflora var.Brookes 48. Brooks 48 is thornless, and has proven itself in Southern Africa over many years. This material was probably obtained from South Africa, as there are close links between growers in the two countries. Most plants that were imported from France and Israel, were budded onto Rosa indica major (referred to 'Indica' ) selections. The variety Natal Briar is currently not widely used in Zimbabwe. All three these rootstocks are propagated vegetatively.
Research that was done many years ago in South Africa, showed that Brookes 48 is tolerant to local
Agrobacterium tumefaciens strains as compared to varieties such as 'Clarke 1957', Basye no. 3' and Manetti'. Brooks 48's resistance to crown gall probably contributed to its popularity in Southern Africa. Recent work done in the Netherlands showed that Natal Briar is resistant to 2 different
A. tumefaciens strains. In the same trial, rootstock varieties such as Indica, Enermis 137 and Popeye showed no resistance to
A tumefaciens as compared to susceptible plants such as tomato and kalanchoe.
The crown gall problem in Zimbabwe was probably the result of the climate and the use of a susceptible rootstock. Agrobacterium infected plants may have been imported during the previous two decades when the growth rate of the industry did not allow for local production.
To solve the crown gall problem, Zimbabwean rose propagators called in researchers of the Agricultural Research Council in South Africa.
Twenty eight rootstock varieties were established in nuclear blocks at Roodeplaat in the 1960's. The first disease-free cuttings were supplied to growers in 1967. The nuclear plants were the source of propagation material in South Africa for many years. Growers did not like to use Natal Briar as it has vicious thorns. Sadly,
growers in South Africa do not contribute financially towards the maintenance of these plants. Plants of Brooks 48, Natal Briar, Manetti and Indica were established in a green house in 1999 and were indexed repeatedly to make sure that they are indeed free of crown gall and relevant viruses. The nuclear plants are used for tissue culture propagation.
Nuclear plants are propagated through tissue culture to make sure that propagation stock remains free of diseases. Rooted plants are planted out in seedling trays in a climate-controlled greenhouse. When plants are 10 cm tall and have strong roots,
they are removed from the seedling trays and packed in boxes for export. On arrival in Zimbabwe, the plants are planted in propagation blocks that conform to the standards of the EFGAZ Rose Plant Improvement Scheme which was specifically developed for EFGAZ. If growers stick to the recommended protocol and procedures such as isolation, renewal of nuclear plants and sanitation of this scheme, disease-free rootstock cuttings can be produced. The first plants were delivered in Zimbabwe in 2000. Growers in Zimbabwe are pleased with the growth rate of the plants.
Fienie Niederwieser, Agricultural Research Council, South Africa.
Email: fienie@vopi.agric.za
Tel: 012 841 9611,(int) 2712 841 9611
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