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

The intensive use of the demethylation inhibitors (DMI fungicides) in recent years has probably led to reduced efficacy of these fungicides in controlling the powdery mildew fungus, Sphaerotheca pannosa (Wallr.) Lev. in nectarine trees in Israel. Field experiments conducted in 1998-2000 showed that foliar applications of DMI fungicides were less effective than the newly introduced strobilurins kresoxim-methyl and trifloxystrobin and the polyoxin B (Polar), in controlling S. pannosa on fruits and leaves of field-grown nectarine trees. However, tank mixing of the DMI fungicides at their recomended use rates, with a reduced rate of sulfur (0.2% v/v) significantly enhanced the inhibitory effect against powdery mildew, in comparison with each component alone at the given rate, and provided the highest level of disease control. These mixtures were even slightly more effective than the sulfur treatment at full-rate (0.5%). Starting the foliar applications of the DMI myclobutanil at the earlier 40-50% bloom stage provided better control of powdery mildew than starting them at petal fall stage, as recommended for this crop, and markedly reduced the disease incidence on fruit and leaves by 77% and 89%, respectively, compared with controls. However, no significant differences in disease incidence were observed between the two regimes when applications of kresoxim-methyl were made. An alternative disease management programme, which integrates an early application of fungicides, and tank-mixtures of DMI fungicides with sulfur, may provide an improved level of powdery mildew control and could be valuable for inclusion into strategies intended to minimize the development of fungicide resistance.