Improved control of powdery mildew (Sphaerotheca pannosa) of nectarines inIsrael using strobilurin and polyoxin B fungicides; mixtures with sulfur; and early bloom applications

The intensive use of the demethylation inhibitors (DMI fungicides) in recen t years has probably led to reduced efficacy of these fungicides in control ling the powdery mildew fungus, Sphaerotheca pannosa (Wallr.) Lev. in necta rine trees in Israel. Field experiments conducted in 1998-2000 showed that foliar applications of DMI fungicides were less effective than the newly in troduced strobilurins kresoxim-methyl and trifloxystrobin and the polyoxin B (Polar), in controlling S. pannosa on fruits and leaves of field-grown ne ctarine trees. However, tank mixing of the DMI fungicides at their recomend ed use rates, with a reduced rate of sulfur (0.2% v/v) significantly enhanc ed the inhibitory effect against powdery mildew, in comparison with each co mponent alone at the given rate, and provided the highest level of disease control. These mixtures were even slightly more effective than the sulfur t reatment at full-rate (0.5%). Starting the foliar applications of the DMI m yclobutanil at the earlier 40-50% bloom stage provided better control of po wdery mildew than starting them at petal fall stage, as recommended for thi s crop, and markedly reduced the disease incidence on fruit and leaves by 7 7% and 89%, respectively, compared with controls. However, no significant d ifferences in disease incidence were observed between the two regimes when applications of kresoxim-methyl were made. An alternative disease managemen t programme, which integrates an early application of fungicides, and tank- mixtures of DMI fungicides with sulfur, may provide an improved level of po wdery mildew control and could be valuable for inclusion into strategies in tended to minimize the development of fungicide resistance. (C) 2001 Elsevi er Science Ltd. All rights reserved.