Farmer's Weekly (South Africa)
Watch out for those aphids!
As a viral infection in your cucurbit crop is such a big problem, not to mention almost inevitable, it’s worth expanding on what I wrote last week.
Usually, on the Highveld, viruses become more common in the New Year, but start much earlier on the Middleveld.
Baby marrows are particularly vulnerable as they are normally picked every other day. This amount of handling can cause a virus to spread very rapidly. It is transmitted through the sap.
An effective precaution is to break the fruit off the plant and trim it in the packhouse. This may seem like extra work, but cutting the fruit off the plant in the land with a knife will increase the speed of infection.
REMEMBER THAT VIRUS RESISTANCE DOES NOT EQUAL IMMUNITY
Baby marrow farmers usually employ successive plantings for harvest continuity over a long period. In doing so, it is only natural to have successive plantings next to one another for easier spraying and other tasks. But again, this makes it far easier for the virus to spread.
Where feasible, space subsequent plantings as far as possible from one another.
Bearing in mind that aphids are the main vector of viruses, carry out the first planting on the leeward side of the farm (that is, downwind of the prevailing winds). The second planting should then be on the windward side (upwind of the prevailing winds). The winged aphids will then be less likely to be blown into the second planting.
With successive plantings, you are also likely to be harvesting from the first batch as the second starts to come into production. And you need to be very careful here.
Avoid harvesting the old planting and then moving the workers straight into the new planting. This will increase the chance of introducing a virus into the second planting.
Because a virus infestation is inevitable at some point, especially when growing marrows or patty pans, many farmers plant early varieties in order to harvest as many fruit as possible before the onset of a virus.
Some farmers even get the pickers to rinse their hands in disinfectant frequently during harvesting to further reduce the chance of a virus spreading. (Milk can also be used as a disinfectant.)
THE LIMITS OF RESISTANCE
Although virus-resistant varieties are available, remember that resistance is not immunity; it merely reduces the severity of symptoms. Therefore don’t be lulled into a false sense of security when planting a virusresistant variety. You should still take every precaution possible to prevent and reduce the spread of viruses.
Don’t wait until you spot aphid damage; inspect the crop frequently, paying particular attention to the underside of leaves and developing buds. Remove damaged plants immediately.
Aphids become infectious immediately after probing an infected plant, but won’t remain so for long, unless they feed for a day or more.
KEEP YOUR EYES OPEN
I cannot overstate the importance of scouting your crop regularly, especially early in the season, when plants are in danger from aphids that have fed on infected weeds.
In short, cucurbit growers need to become very aphid-conscious! • Bill Kerr is a vegetable specialist and a breeder of a range of vegetables. Email him at farmersweekly@ caxton.co.za. Subject line: Vegetable production.