b'TATERTALESRevisiting the Impact of HeatStress on PotatoesHeat during the growing season is causing disease problems for potato growers. HOT, DRY SUMMERS have become more prevalent in Ontario and elsewhere during the last four years. Potato productivity is greatly reduced at high temperatures, above 30 C during the day and 25 C at night. High temperatures reduce yield, cause secondary tuberization and increase physiological defects on tubers. The effect will depend on the variety and the stage of growth, but in general the earlier a heat wave occurs, the more negative its impact.High temperatures during tuber initiation increase the EUGENIA BANKS,respiration rate using up energy and reducing tuber set. A high POTATOrespiration rate mid-season or later may cause the production of SPECIALIST WITH THE ONTARIOa second set of tubers which never size uphigh temperatures POTATO BOARD during bulking slow down tuber bulking resulting in yield losses.Important physiological disorders caused by high tempera-tures are: Tuber deformations: Deformations are primarily caused by high temperatures which stimulate cell division and reduce the availability of carbohydrates by increasing respiration. Water stress does not in itself cause deformations, but drought exacerbates the deformations caused by high temperatures. TheInternal discolouration was detected in these tubers during harvest last year.PHOTO: EUGENIA BANKShigher the temperatures and the longer the heat wave, the greater the effect. Internal Brown Spot (IBS): Light brown necrotic spots will Examples of tuber malformations are dumbbell shaped tubers,develop anywhere in the tuber flesh at the latter stages of tuber pointed bud-end or pointed stem-end tubers and kidney-shapedgrowth. IBS is often due to an enzymatic disorder caused by tubers. Knobby tubers are produced when the apical dominanceintense heat or excessive drought. It has also been associated of the bud end is reduced resulting in secondary growth at thewith calcium deficiency.lateral eyes of the tuber.Second Growth: Potato plants respond to temperatures of 30 CTranslucent End and Jelly End Rot: High temperature andand above by increasing vegetative growth rather than setting tubers water stress during the early stages of tuber growth may interfereor bulking tubers. This can result in heat sprouts and chain tubers or with starch deposition at the stem end of the tuber. Often the tis- heat runners in susceptible varieties.sue of tubers affected with Translucent End breaks downthis Blackheart: Any pre-harvest, transit or storage conditions breakdown is called Jelly End Rot. Usually pointy bud-end tuberswhich prevent oxygen from reaching the center of the tuber will develop Jelly End Rot. result in blackheart. Long exposure of tubers to field temperaturesGrowth Cracks: Growth cracks develop when relativelyabove 32 C before harvest usually result in blackheart.poor growing conditions switch rapidly to relatively good growing Physiologically old tubers: Heat stress increases physiologi-conditionsthis could be hot, dry weather followed by excessivecal age drastically. Physiologically old tubers tend to sprout early, irrigation or heavy rainfall and a drop in temperatures. produce more stems, and cause little potato formation defect. Heat Necrosis: Heat necrosis occurs when a period of slowCalcium fertilization with a water-soluble form of calcium tuber growth is followed by active tuber growth at high tempera- during bulking can mitigate the adverse impact of heat stress tures. Light to dark brown necrotic spots form in the vascularon tuber yield. The best way to apply calcium during the growing tissue of the tubers. The varieties Chieftain and Atlantic are veryseason is to side dress Ca(NO3)2 and then irrigate, according to susceptible to heat necrosis. Jiwan Palta at the University of Wisconsin. 62 SPUDSMART.COM WINTER 2021'