Articles
VIRUS PROBLEMS IN PROTECTED VEGETABLE CROPS
The environmental conditions and cultural practices in protected crops are notably different from those in open fields and this may cause the same vegetable, under the two conditions, to be affected by diverse virological problems.
In particular, viruses which are ecologically adapted to tropical climates, e.g. tomato spotted wilt and cucumber green mottle mosaic, may become a serious problem under glass in temperate and cold climates.
The conditions of protected crops are particularly favourable to the establishment and spread of highly infective viruses which persist in the soil and spread by contact from plant to plant.
The most typical examples are tobacco mosaic and a few other tobamoviruses.
As far as viruses which are vectored by insects or other organisms are concerned, plants in protected crops are less exposed directly to airborne vectors from outside.
The incidence of non-persistent aphidborne viruses, in particular, is generally much lower in protected than in open fields.
Some problems with these viruses may arise when they are carried in by seed or when typical glasshouse-infesting aphids are involved as vectors.
Lettuce mosaic, which is seed-borne in lettuce, and Aphis gossypii as a vector of cucumber mosaic may be taken as examples.
The whitefly Trialeurodes vaporariorum may be considered the most efficient insect vector of viruses in glasshouses.
It is very difficult to control, and transmits cucumber yellows, beet pseudo-yellows and other viruses causing yellowing type symptoms in vegetabls.
Soil-borne viruses and virus-like agents can affect either open or protected crops at similar levels.
Some cultural practices which are used in protected crops may favour the spread of infection, as is the case for the hydroponic culturing of lettuce in respect of the fungus-borne lettuce big vein agent.