Viral diseases are pretty deadly, once a plant becomes infected. There are no control measures other than to slow down the spread to other plants and land.
Viral diseases are pretty deadly, once a plant becomes infected. There are no control measures other than to slow down the spread to other plants and land.
There are four main viruses to contend with and it’s not easy for the farmer to distinguish between them. They are: WMV 2 (watermelon mosaic virus 2) ZYMV (zucchini yellow mosaic virus) PRSV (papaya ringspot virus) and CMV (cucumber mosaic virus).
To complicate matters further, the viruses sometimes combine – a plant may be infected with two different ones which are transmitted together by the aphid vector.
Certain viruses are more problematic in certain areas but more often than not, it will depend on which virus started off in any area in the beginning of the season.
Outdoor cucumbers, which include the US slicers and picklers, are often virus-resistant as are new varieties of baby marrows. Some baby marrows have a virus tolerance which clearly manifests in infected plants being much more healthy than susceptible counterparts. But the fruit becomes blemished and unmarketable, which makes the resistance of little commercial benefit. However, new varieties have meaningful resistance.
Our pumpkins, Cucurbita maxima, don’t have virus resistance although the resistant genes are available, no-one has gone to the expense and effort of breeding them into a variety. Ironically, some of the US Halloween pumpkins, C. pepo, have been bred to have resistance. They are the same species as our gems and baby marrows and will readily cross. We often see someone showing off a giant gem which is actually a cross between a gem and Halloween pumpkin.
Viruses cause distortions in the leaf growth of the crop as well as lumps on the fruit, which become unmarketable and much smaller. If the plant is infected young, it’s a write-off. On the Highveld, the season usually starts off clean but later on, towards January, symptoms can be found on the odd plant. The Middleveld starts off much earlier, while warm areas can start any time depending on where the infection comes from.
Aphids are the main vector, but I suspect that pumpkin flies can also play a part. Aphids can move vast distances in the wind to spread the virus. In the case of baby marrows, the virus can also be spread mechanically by workers, especially if they are cut off by knives. The only control measures are to keep the vectors under control and to take out the infected plants as soon as they show up, but this usually only slows the disease down. – Bill Kerr ((016) 366 0616 or e-mail [email protected]). |fw
