WHY VINE DESICCATE? WHEN TO VINE KILL?
Natural vine desiccation occurs when plants die because the variety is determinate, that is, it is genetically programmed to do so after a length of time. Determinate varieties tend to be short to mid-season varieties but can be long or full-season ones as well. Branching in these varieties ends with flower formation. Indeterminate varieties such as Russet Burbanks tend to continue branching after flowering even thought at a slower growth rate. Environmentally-induced vine desiccation is caused by a change in the weather, primarily cold temperatures as fall progresses. A night freeze will induce dying of leaves and then the whole plant. However, this may not occur until mid to late autumn (October and November).
If plants will die eventually either due to age or cold, why desiccate them mechanically and/or chemically? There are two primary reasons: eliminate vines and enhance tuber maturation.
Eliminate Vines
Eliminating vines either by physical removal, burning or killing will stop bulking or growth of the tubers. It will promote the separation of tubers from rhizomes (stolons). Harvest is made easier with less wear on machinery parts. Most importantly, it will reduce the danger of vines spreading pathogens from vines to tubers and reduce disease occurrence in storage. Key diseases of concern are late blight, early blight, bacterial soft rot, viruses such as leaf roll virus causing net necrosis, and others.
Tuber Maturity
For tubers to store well and for processing into chips and fries, they need to mature. Vine kill stops nutrient flow from the vine into the tuber. The skin will set, that is, thicken and harden, thereby reducing bruising such as skinning and shatter. This also reduces the danger of diseases in storage as many pathogens enter tubers through breaks in the skin. With vines dead, the tubers can maintain their dry matter content, such as starch, protein and vitamins, and maintain a sugar (glucose and sucrose) level acceptable for frying to make chips and fries. It takes about three weeks after vine death, depending on conditions, for tuber skins to set and maturation to complete (Pavlista, 2002).
Timing and Conditions
Vine desiccation occurs at the end of the growing season after the log phase of tuber bulking. With fall harvest, that is at the end of August to the end of September or about 14-16 weeks after planting. The soil moisture should be about 65% field capacity and little, if any, irrigation is needed. To avoid soil clumping, sometime some irrigation is given. No fertilization, specifically, nitrogen is desired although some sulfur, as ammonium sulfate or sulfuric acid, application can enhance vine desiccation. Nitrogen application late in the season can delay vine senescence, inhibit desiccation and promote regrowth of new branches.
Reference
- Pavlista, AD. 2002. Skin set evaluation by skin shear measurements. Am Jour Potato Res 79:301-307.
Vine Desiccation: