Management of Cylindrocarpon black foot disease in New Zealand nurseries and vineyards

Black foot disease of grapevines, a significant cause of vine death in New Zealand and world-wide, is difficult to control in nurseries and vineyards. This research investigated effects of fungicides, hot water treatment (HWT) and mustard biofumigation to control Cylindrocarpon black foot disease. These methods were tested against nine isolates, three each of the three Cylindrocarpon species; C. destructans (Ilyonectria radicicola species complex), C. liriodendri and C. macrodidymum (Ilyonectria macrodidyma complex), isolated from symptomatic grapevines from New Zealand vineyards and nurseries, in vitro and in vivo. The in vitro testing of 12 fungicides showed that captan, carbendazim and didecyldimethyl-ammonium chlorine effectively inhibited conidium germination and mycelium growth of all nine isolates with EC50 values of 1.0-150, 0.05-100 and 1-1000 (mg a.i./L), respectively. Cyprodinil + fludioxonil, fludioxonil and tebuconazole were similarly effective at inhibiting mycelium growth (P≤0.001) and conidium germination (P≤0.001) of most isolates. In two field sites, the fungicides significantly reduced disease severity (P≤0.001) and incidence (P≤0.001) in rootstocks 101-14 and 5C, with captan and carbendazim + flusilazole being effective in Auckland and cyprodinil + fludioxonil and Tricho-Flow™ effective in Blenheim. These products could therefore be used as preplanting dips and didecyldimethyl-ammonium chlorine (a sanitiser), could be used in nursery systems for prophylactic control of black foot pathogens on propagation material. Also tested, HWT (47°C for 30 min), was the most effective treatment against the black foot pathogens at both sites (P≤0.001). The in vitro testing of different HWT protocols on mycelium growth, conidium germination and mycelium within grapevine canes showed that conidium germination was completely inhibited (P<0.001) by heat treatments greater than 40°C for 5 min, and that mycelium growth was inhibited (P≤0.003) for all but one isolate by treatments greater than 47°C for 30 min. Within cane pieces, all Cylindrocarpon isolates were inhibited (P<0.001; 0.0 to 0.9% incidences) by 30 min at 48.5 and 50°C. In field trials, HWT protocols of 48.5 and 50°C for 30 min significantly reduced disease severity (P≤0.001) and incidence (P≤0.001) to 0%. This confirmed that HWT of 48.5°C for 30 min could replace the industry standard HWT of 50°C for 30 min to eliminate black foot disease in nursery grapevines in New Zealand. Physiological effects on bud development and carbohydrate concentrations were examined after HWT, cold storage and different dates of harvesting dormant plants. HWT reduced disease incidence (P≤0.010) and severity (P≤0.001) as did later (July) harvest dates (P=0.010 and P=0.002, respectively), although cold storage had no significant effect (P>0.05) on disease levels. Over the months of harvest, root sugar concentrations were relatively constant (P=0.023) whereas starch levels were reduced (P<0.001). Cold storage increased sugar (P≤0.001) and reduced starch concentrations (P≤0.001) although the effect was less for HWT plants. Growth stages E-L 4 and E-L 9 were reached more slowly with plants harvested earliest and more quickly for HWT and cold stored plants (all P<0.001). Variety effects (P<0.001) showed that grafted grapevines 101-14 cv. Pinot noir reached growth stage E-L 9 most quickly, followed by 5C cv. Sauvignon blanc and then 101-14 cv. Sauvignon blanc. Results showed that if grapevines are not harvested in deep dormancy, cold storage for four weeks prior to HWT could improve rate and uniformity of bud development and reduce disease incidence. In vitro biofumigation with Brassica juncea inhibited conidial germination and colony development, and mycelium growth of all Cylindrocarpon isolates (all P