Introduction
There are different types of passion fruits in Uganda namely : small purple (Passiflora edulis f. edulis); yellow (P. edulis f. flavicarpa), hard shell (P. maliformis), giant passion fruit (P. quadrangularis) sweet granadilla (P. lilagularis) and Kawanda hybrid ( a cross between small purple and yellow type). The commonly grown passion fruits are the small purple, yellow and Kawanda hybrid. Kawanda hybrids clones 5, 6, 6/7 and 8 which were released in 1985 became popular commercial passion fruits because of their big fruits, high yields and resistance to some foliar disease. However most of these clones have segregated into a new set of unknown populations in terms of agronomic requirements, resistance to pests and disease and production.
Passion fruit production and marketing in Uganda has been declining in the past few years mainly due to pests and diseases. The main killer diseases are collar rot and woodiness virus disease complex.
a). Collar rot disease
This is a disease caused by a fungus known as Fusarium solani. The affected passion fruit plants show symptoms of dark spots on the upper parts and girdling at the base of the stem just at ground level. The girdled area exhibit a rotten tissue which affects and interferes with the food and water translocation in the plant. This eventually leads to the withering, a situation termed wilting, and to subsequent drying and death of the infected plant. The disease causes severe symptoms especially on plants that are in their productive stage and therefore demanding a lot water from the soil. Affected plants usually last for a few weeks from the time of initial symptoms. The disease is mostly spread though contaminated soil and infected plants.
Like most soil borne diseases, there is no effective chemical control for the disease. Grafting technique has been proved useless since the yellow passion fruit which is used as rootstock succumbs to the disease. Passion fruit production is recommended for unaffected areas.
b). Woodiness virus disease
The disease is caused by a virus that belongs to potyvirus group. The disease affects plants at all stages from nursery to mature plants. Common symptoms include exhibition of yellowish distorted plant parts such as shapeless, small sized and clucked leaves. The entire affected plant or branches can be stunted. Affected fruits become very hard like a stone and are usually deformed, shapeless and under size. The disease is spread by sap sucking insects such as aphids and mealy bugs, and mites. Woodiness virus disease is also commonly transmitted through vegetative propagation practices such as grafting of infected scions and use of contaminated tools. Plant nurseries have mushroomed country wide but most of these are not inspected and some nursery owners lack the know-how and therefore sell fake and often diseased planting materials. This has greatly aggravated the viral disease problem.
Like all viral diseases, there is no chemical control for woodiness virus. Disease avoidance through the use of clean planting materials and chemical control of vectors can assist to reduce the spread and effect of the disease.
2. Previous work on passion fruit
Research work has been conducted on collar rot, woodiness virus and pests such as aphids, mealy bugs and mites. Results indicated that all the sampled yellow passion fruit, which is used as rootstock, was susceptible to collar rot infection. All the types of passion fruit such as hard shell, yellow, small purple and Kawanda hybrid were susceptible to virus attack. However, research work also indicated that the viruses are not seed borne. The effect and role of pests on passion fruit production has not been fully elaborated as there seems to be more pests than the mentioned ones.
3. Research Achievements
Concerted research efforts have been made and continue to be made to contain the problem emanating from the two major diseases but with little success due to limited human and financial resources. However, the following have been achieved at the Horticulture Research Programme
Yellow passion fruits clones with high productivity potential and tolerant to collar rot disease have been collected and evaluated. Crossing to form a gene pool is on-going.
Back- crossing of Kawanda hybrid to reduce the acidity level in progress.
Kawanda hybrid clones tolerant to viral diseases collected and planted.
Molecular characterization of the virus causing the passion fruit woodiness disease has been initiated with successful isolation of the virus RNA and characterizing it as that of a potyvirus.
4. On going research
As mentioned above, Passion fruit woodiness disease is continuing to devastate farmers’ fields. There is a great desire among the farmers to find a control strategy for this disease. Nevertheless, it may be very hard to find a control strategy before the spread dynamics, transmission and potential vectors of the disease is established. For this reason, the Horticulture research program is undertaking research to establish the epidemiology of the passion fruit woodiness disease. A trial has been setup at National crops resource research institute at Namulonge. The aim of the research is to establish the special and temporal spread, transmission and species composition and abundance of potential vectors of passion fruit woodiness disease