November 2004. A problem of virus-like symptoms on Snake bean in Papua New Guinea. In this case from Port Moresby. The leaves turn yellow and curl inwards at the edges. The leaf also broke easily when tapped at the decolourised region. Numerous homopterans were feeding on the plants, and it was wondered if these could be the vectors.
In Thailand, it was Cowpea-aphid borne virus (?Blackeye cowpea mosaic – Editor), which is seed transmitted. Seeds are tested before they are released to the market.
The virus could not be identified from the symptoms. As a general rule, symptoms on young leaves are likely to be the result of virus infection. However, with legumes, there are a large number of possiblities.
Little is known of the plant virus situation in PNG. Surveys have been done along the border with Irian Jaya, and virus-like symptoms, often at high incidences, have been found in legume weeds and sometimes legume crops. Legumes are likely to be infected by several potyviruses (a large group of plant-infecting viruses), many of which are seed borne at relatively high rates. The results of the surveys have been written up in Australasian Plant Pathology, which confirmed potyvirus infection in several species, plus one record of Glycine mosaic virus (a Comovirus). Some molecular studies are ongoing.
So in summary: (i) the symptoms are certainly very typical of those associated with virus infection and (ii) the exact virus is difficult to determine because legumes are susceptible to a number of viruses. It is possible that a potyvirus such as Bean common mosaic virus is the culprit – this virus has been reported from Guam infecting snake bean (Vigna unguiculata ssp. sesquipedalis) and also from a range of legumes in Solomon Islands. It is transmitted by seed and aphids. The best method of control is to remove the infected plants (and bury them) because they serve as a source of infection for neighbouring plants.
However, it shoud be remembered that plant viruses should never be diagnosed from symptoms alone. Visual diagnostics of diseases caused by intracellular pathogens (viruses and virus-like agents) based on plant symptoms is a practice that used to happen in the old days, when there was often no choice, but there is no excuse for it in 2004!