Factors Influencing the Efficacy of Myclobutanil and Azoxystrobin for Control of Grape Black Rot

Abstract

We studied several factors influencing the efficacy of the demethylation inhibitor (DMI) fungicide myclobutanil and the strobilurin fungicide azoxystrobin for control of grape black rot, caused by the pathogen Guignardia bidwellii (anamorph Phyllosticta ampelicida). The distribution of sensitivities to myclobutanil among G. bidwellii isolates from an “organic” vineyard (no previous exposure to synthetic fungicides, n = 50) and from a commercial vineyard with a history of DMI applications (n = 60) was determined in vitro. There was little difference between the two populations, and the range of sensitivities was narrow; for the composite population of 110 isolates, the value of the mean effective dose for 50% inhibition (ED50) was 0.04 mg/liter, and the most- and least-sensitive isolates were separated by a factor of 16. When applied from 2 to 6 days after inoculating grape seedlings with a suspension containing either 2 × 104 or 1 × 106 conidia per ml, myclobutanil (60 mg/liter) provided complete control of lesion development. When applied beyond 6 days after inoculation but prior to lesion appearance (9 to 11 days after inoculation, depending on temperature), it provided complete control of pycnidium production in those lesions that developed subsequently. In contrast, when applied 2 to 10 days after inoculation with 2 × 104 conidia per ml, azoxystrobin (128 mg/liter) provided only 78 to 63% control of lesion formation and erratic control of pycnidium formation, although conidium production was reduced by 85 to 68% across this range of treatments. Relatively little control was provided by azoxystrobin treatments following inoculation with 1 × 106 conidia per ml. On leaf disks treated with azoxystrobin at 20 mg/liter prior to inoculation, 8 to 43% of conidia from five G. bidwellii isolates germinated, and 4 to 19% formed appressoria. However, these processes were completely to near-completely inhibited when salicylhydroxamic acid (SHAM), which inhibits an alternative respiration pathway utilized to circumvent the activity of strobilurin fungicides, was added to the inoculum at 100 mg/liter. Thus, alternative respiration apparently allowed the conidia to germinate and form appressoria on azoxystrobin-treated leaves. When grape seedlings were sprayed with commercially formulated azoxystrobin at 200 mg/liter and inoculated the next day with G. bidwellii conidia, little or no disease was evident 4 weeks later. However, G. bidwellii pycnidia formed on up to 50% of the leaves from such plants when they were killed with paraquat 1 to 7 days after inoculation. These results suggest that latent infections became established on azoxystrobin-treated leaves and became active after the plants were killed with paraquat.

Effect of Azoxystrobin 8.3%w/w + Mancozeb 68.75% on Powdery and Downy mildew Pathogens under In vitro condition

“…Similarly, Azoxystrobin (23 SC) recorded cent per cent inhibition of conidial germination at a concentration of 250 ppm and above. Azoxystrobin provided cent per cent control of downy mildew in grapes, when applied 1 to 5 days before inoculation and 85 per cent mean reduction of resporulation from diseased tissue, when applied 6 days after inoculation (Wong and Wicox, 2001) [17]. Azoxystrobin inhibited the conidia to germinate and formation of appressoria in black rot (Guiganardia bidwelli ) pathogen of grape (Hoffman and Wilcox, 2003) [18].…”

Curative and Eradicative Effects of Fungicides

“…Propiconazole, tebuconazole, myclobutanil, flusilazole, triforine, fenarimol, vinclozoline and iprodione were very effective in reducing blossom blight when applied 48 hours after inoculation of sour cherry with 5000 Monilinia fructicola conidia, but they were less effective when plants were inoculated with 50 000 conidia (Wilcox, 1990). Postinfection disease control with azoxystrobin was low when grapevine seedlings were inoculated with million Guignardia bidwellii conidia/ml, while it was relatively high when plants were inoculated with 20 000 conidia/ml (Hoffman & Wilcox, 2003). Effectiveness of curative treatments of tebuconazole, cyproconazole, epoxiconazole, flusilazole and prochloraz in control of Septoria tritici on wheat was different depending on the environmental factors which have conditioned disease development in different experiments (Schöfl & Zinkernagel, 1997).…”

“…After treatments of grapevine seedlings inoculated with Guignardia bidwellii, myclobutanil and azoxystrobin suppressed formation of pycnidia in lesions caused by the fungus (Hoffman & Wilcox, 2003). Application of myclobutanil resulted in 100 %, 91 % and 75 % reduction of Guignardia bidwellii pycnidium formation when applied six, eight or ten days after inoculation of plants with the fungus (Hoffman & Wilcox, 2003). Trifloxystrobin significantly reduced sporulation of Fulvia fulva on tomato leaves when applied on symptomatic plants (Veloukas et al, 2007).…”

“…Higher levels of disease control was achieved with difenconazole comparing it to azoxystrobin in curative treatments of sugar beet inoculated with Cercospora beticola (Anesiadis et al, 2003), and the same was recorded comparing curative effects of tebuconazole, myclobutanil, pyraclostrobin and trifloxystrobin on Diplocarpon rosae (Gachomo et al, 2009). When applied to grapevine seedlings inoculated with Guignardia bidwellii, myclobutanil provided complete control of lesion development up to six days after inoculation, while azoxystrobin provided only 61 % control of lesion formation (Hoffman & Wilcox, 2003). On the other hand, there are examples of other diseases where better results in postinfection treatments were obtained with strobilurin fungicides than with triazoles.…”

“…When propiconazole and azoxystrobin were applied to ryegrass plants infected with Puccinia graminis, they reduced urediniospore production by 73 % and 95 %, respectively (Pfender, 2006). After treatments of grapevine seedlings inoculated with Guignardia bidwellii, myclobutanil and azoxystrobin suppressed formation of pycnidia in lesions caused by the fungus (Hoffman & Wilcox, 2003). Application of myclobutanil resulted in 100 %, 91 % and 75 % reduction of Guignardia bidwellii pycnidium formation when applied six, eight or ten days after inoculation of plants with the fungus (Hoffman & Wilcox, 2003).…”