[deck]Desiccation offers plenty of benefits at harvest—if growers play their cards right.[/deck]
Vine killing prior to harvest, also called desiccation, is a common practice on North American potato farms. Vine killing should be done well before harvest, usually two to three weeks. By desiccating when optimum harvest conditions have been reached, growers can control many of the variables that adversely affect the quality and yield of the crop. Desiccating potato vines is essential for several reasons: it ensures good tuber separation from stolons during harvest, allows for sufficient tuber skin set, and aids the overall efficiency of harvest.
According to Khalil Al-Mughrabi, pathologist at the Potato Development Centre of the New Brunswick Department of Agriculture, Aquaculture and Fisheries, vine killing is a harvest aid used to reduce losses from late blight tuber rot, minimize virus infection spread by aphids, control tuber size and reduce skinning tubers during harvest. Al-Mughrabi points out that vine killers are either applied at least 14 days prior to harvest, or whenever the tubers have sized sufficiently.
Tubers from immature or recently-killed vines are typically highly susceptible to skinning as well as mechanical injury during harvest. These tubers often create problems during storage due to early decay. Immature tubers also tend to have relatively low starch and high sugar concentrations compared to mature tubers, which causes difficulties achieving good fry colour during processing.
Al-Mughrabi says that for table and processing crops, a relatively slow—but complete—kill of the potato leaves and vines is most desirable. “This type of kill will allow the tubers to continue sizing while still providing the conditions necessary for good skin set,” he points out.
“A slow kill also allows the tubers to continue converting sugars to starch, [which is] vital to good processing quality. Research in New Brunswick has shown that split applications of chemical desiccation result in better-coloured french fries out of storage than a single, full-dose application of a chemical product.”
Several vine removal methods are available to growers. Options include chemical sprays, mechanical methods such as chopping and rotobeating, or even, in rare cases, flaming of the vines with specialized flaming equipment. Growers often use a combination of methods.
According to Al-Mughrabi, rotobeating or chopping results in an almost immediate halt to tuber growth. As a result, he advises growers to treat the stems with a chemical vine killing product to ensure that the vines don’t act as a source for blight spores. It is recommended that growers continue to apply protectant sprays against late blight until the vines are completely dead.
Under normal weather conditions, a split application of a chemical vine killing product should produce the desired results. Al-Mughrabi recommends an application of one-third to two-thirds of the full dose, followed in four to six days with the remaining amount of the recommended dose of the chemical product. During seasons where wet weather combined with a high rate of fertilization has resulted in lush green tops, a full-dose application may be needed during the first application.
When the crop is intended for seed, a faster kill may be needed to control size and minimize disease spread by aphids. Al-Mughrabi advises that seed growers always apply the full label rate to obtain a faster kill.
Reglone, which contains the active ingredient diquat dibromide, is one of the most commonly used potato desiccants in Canada. The group 22 non-selective contact herbicide interferes with photosynthesis, causing cell damage which in turn leads to desiccation.
According to information provided on the Reglone label, a sprayer fitted with flat fan or hollow cone nozzles operating at 150 to 200 kPa is preferred to ensure even coverage and minimize drift. Additionally, the boom should be set above the crop at such a height that there is a complete double overlap of the spray pattern.
Use of chemical topkillers has been suggested as one cause of stem-end discoloration. Also, the practice of rotobeating tends to reduce yields and quality, including colour, and the sudden shock to the potato plant may increase the amount of stem-end discolouration.
However, according to Al-Mughrabi, there are other factors that play into discolouration. “Usually the plants have to be under some form of stress, such as drought, for stem-end browning to occur,” he says. Serious drought sometimes occurs in Atlantic Canada and portions of the Atlantic region can be very dry in August. He points out that the use of a higher-than-recommended rate of vine killer can also produce discolouration.
The combination of drought and excessive use of a desiccant product is almost sure to cause problems for growers. Under drought conditions growers are advised to delay desiccant application until the soil is moistened by rainfall or irrigation. Some vine killers have specific requirements, so growers should always read and follow the product label directions carefully.
Technical and Environmental Considerations
Growers have been successfully applying vine killing treatments for several years at low pressures of 100 to 200 kPa using sprayers equipped with fan-type nozzles. This is the same pressure used for spraying herbicides—and vine killers are indeed herbicides.
On the other hand, high pressures can cause problems. “The use of high pressure, 700 kPa or higher, is dangerous to the health of the spray operator and increases the drift of chemical from the field,” says Al-Mughrabi. When spraying conditions are not ideal, this loss of chemical results in poor vine killing.
Growers should always read the label and use the water volume recommended to ensure vine killing can occur without harm to the spray operator, the environment—and the crop.