There are several reasons for desiccating vines chemically as opposed to mechanically and there are several products available.
Using a chemical product will kill vines, leaves, branches and stem. This makes harvesting much easier and cleaner of debris. Since chemical vine desiccation with the exception of sulfuric acid takes time, usually three weeks, to complete, tuber maturity and skin set has time to develop and complete. Very importantly, this is the best method to control disease spread.
Advantages are that using desiccants stops growth, reduces foliar mass and protects tubers. Bruising does not occur as it often does with mechanical desiccation.
Problems do exist. The major one is consumer fears of potential residue. Chemical desiccation costs more than mechanical. Weather conditions, even time of day of spraying, can cause erratic results.
Diquat -- Possibly the most common and effective desiccant used is diquat dibromide, commonly called diquat. Diquat has been successfully used for a number of decades. It is used throughout the world under product names such as Regulone and Diquat. It works best when vines show signs of senescence. On large vines, sometimes two applications, a week apart, is required. Diquat is at times used in conjunction with vine beating. Either, the vine tops are beaten first to eliminate the upper half to two-thirds of the plant and then shortly afterwards diquat is applied. Or, diquat is applied first to kill the leaves and weaken the stem, and then about two weeks later, the vines are beaten to shred any slow dying stems and regrowth.
Paraquat -- A close relative of diquat, paraquat dichlor has been used even longer than diquat. It works somewhat better than diquat when used on younger, growing plants but there have been decay problems with storing potato tubers from vines desiccated with paraquat. This has restricted labeled use of the material.
Endothall / Hydrothol -- Endothall is a mixture of two salts and hydrothol is one of these salts of endothall formulated alone. The product names of this desiccant are Des-I-Cate and Flair, and are more recent chemicals. In Des-I-Cate, ammonium sulfate has been added to increase desiccation effectiveness but in some situations, this may also promote regrowth (Pavlista, 2001).
Sulfuric Acid -- Sulfuric acid, which can be used as a sulfur fertilizer or to acidify alkaline soils, needs to be applied with very specialized equipment. It is dangerous to the applicator and specially licensing is often required. The acid, which has a very low pH in its concentrated form, is applied at high concentrations, around 90%, directly to plants. Sulfuric acid can be used unchanged or as a product that release the acid such as Enquik. Products such as Enquik are not quite as dangerous as straight sulfuric acid. Its advantages are a very quick kill and the ability to kill pathogen spores, especially Phytophthora infestans (late blight), on contact at the soil surface. However, pathogens that are not directly in contact with sulfuric acid, such as being under debris, will not be affected. Sulfuric acid is very short lasting as it will be diluted and neutralized by the soil. The vine kill is so quick that the tubers will not mature nor set skin, and vascular discoloration is common.
- Pavlista, AD. 2001. Hydrothol as a vine desiccant of 'Atlantic' potatoes. J Vegetable Crop Production 7:59-68.