f.Other viruses may also be involved and symptoms may be masked.
g.There is an occurrence (red leaf) in Barlinka where the younger leaves change colour first. The connection with leafroll has not yet been established.
h.All present were in agreement that the disease is widespread.
i.Plant material plays a significant role in the distribution of the disease. Regrowth canes from rootstocks are sometimes used and if the rootstocks were infected with leafroll, the disease may spread in this way.
j.Certain methods of detection are not sensitive enough. With rootstocks in particular it is difficult to trace leafroll using ELISA.
k.Plant material that is free of leafroll is distributed to the industry, only to be reinfected with leafroll shortly after planting. Vectors that have already been identified as carriers of leafroll, include three mealybug species and one scale insect species. Other possible vectors that were mentioned, but which have yet to be proven carriers of leafroll, are white flies and aphids. Other unlikely vectors which were nevertheless mentioned are thrips, snails, snout beetles, nematodes and fungi. Considering the feeding habits of mites, it is unlikely that they could be carriers of the virus. At the recent Virus Congress it was reported that Rupestris stem pitting may be carried by pollen.
l.Mechanical transfer of the virus through pruning scissors, root pruning and deep delving could possibly also be a distribution mechanism.
m.Mealybug movement may be influenced by various factors, for example: birds, wind, implements such as harvesters and topping machines and people. Cover crops, weeds and windbreaks could possibly provide a means of survival for mealy bug. Effective combating of ants may reduce mealybug activity. In table grapes it has been observed that the northern end vines in north-south rows usually have leafroll. The occurrence of ants in kraals may also indicate virus distribution.
From the above it is clear that the problem of leafroll is very complex.
3. DISCIPLINES INVOLVED IN THE PROBLEM
The team came to the conclusion that the following disciplines were involved in the study of the leafroll problem:
- Soil science
- Cover crop and weed science
- Molecular biology
- Plant biotechnology
4. SCOPE AND NATURE OF THE INVESTIGATION
The scope and nature of the investigation are summarised as follows:
a.The problem is accepted.
b.The leafroll problem is quite prevalent and it is definitely spreading. In certain areas distribution is more widespread than elsewhere.
c.The investigation should focus on the distribution factors.
d.Short and long term solutions must be found.
e.Projects must be identified.
f.Guidelines for the projects should be drawn up.
g.Researchers or bodies that could possibly do the work, have to be identified.
h.The researchers will be asked to put together projects and submit proposals for funding.
5. SECONDARY GOALS OR PROJECTS
Certain aspects will not be formal projects, but guidelines or a protocol will suffice. The certification scheme should be reconsidered as well as the handling of material in the scheme. The Wine Grape Improvement Association will be requested to draw up guidelines. Another aspect requiring attention is technology transfer. The complexity of the leafroll problem should be explained to the producers so as to obtain more co-operation from them. Technology transfer to consultants is also emphasised.
The following potential projects are identified:
a.Determining the virus status of material in production blocks that have already passed through the certification scheme.
The study will be conducted in nucleus, mother and foundation blocks.
b.Development of leafroll 3 resistant wine grape cultivar or rootstock.
c.Development of more sensitive commercial detection methods for rootstocks that are also practical to apply.
d.Selection of vines that are visually symptom free in infected vineyards.
e.Non-conventional management of mealybug.
f.Documentation and interpretation of the distribution pattern of leafroll.
g.Observation and analysis of factors involved in the distribution of leafroll.
h.Alternative hosts for vectors.
i.Role of mechanisation in distribution of the vector.
j.Basic research on leafroll 3 virus.
k.Other possible vectors of leafroll 3.
l.Can pollen transmit leafroll
m.Evaluation of the standard procedures in the plant improvement scheme.
n.Possible reinfection with leafroll in nurseries.
o.Cultivar sensitivity to leafroll reinfection.
p.Transfer of naturally resistant genes.
6. FURTHER HANDLING OF THE MATTER
The Winetech Committee will prioritise the projects and proposals will be solicited from research bodies. Bodies will have three months to prepare project proposals