‘Noiret' Winegrape Bud Cold Hardiness: Effects of Temperature, Month, and Black Rot Severity

Black rot infections during the summer and bud cold hardiness in the winter are both constant management issues in Oklahoma viticulture. Black rot infections could be a cause of vine stress, leaf drop, and rotten fruit, potentially compromising overall vine ability to prepare for cold temperatures. However, no studies have targeted bud cold hardiness in relation to severity of previous season black rot infection.  Thus, in 2012, ‘Noiret’, a hybrid winegrape, was subjected to four black rot control treatments (conventional (C), organic 1 (O1), organic 2 (O2), and no spray (N)).  Leaves and fruit were scored in late June for lesion severity.  The O1 and N treatments had the highest level of leaf and fruit lesion severity and were not significantly different.  The C treatment had the least amount of leaf and fruit lesion severity and the O2 treatment was intermediate and significantly different from the O1 and N grouping, as well as the C treatment.  After the acclimation period in fall and early winter, bud samples were taken in January and March and exposed to subzero temperatures (-5°C, -10°C, -15°C, -20°C) in an ethylene glycol bath to assess if previous season black rot infection had any impact on bud hardiness.  Five bud samples replicated four times for each month and temperatures were taken. A significant Month * Temperature interaction indicated the number of live buds decreased as temperature decreased.  At -5°C and -10°C in January nearly all buds were still alive (4.9 for both), and not significantly different from March -5°C, -10°C, and -15°C.  Lower temperatures of -20°C in March (3.6) and January (3.0) and -15°C in January (3.5) were not significantly different.  The black rot control treatments were not a statistically significant factor in the bud hardiness experiment.  This could be due to black rot severity being below a critical threshold for impact or that the vines had enough time to recover in late summer and fall to reach full mid-winter hardiness.