Impact of Application Rate and Interval on the Control of Powdery Mildew and Cercospora Leaf Spot on Bigleaf Hydrangea with Azoxystrobin

Abstract

Efficacy of azoxystrobin (Heritage 50W™) was assessed over a range of application rates and intervals for the control of powdery mildew (Erysiphe polygoni) and Cercospora leaf spot (Cercospora hydrangea) on bigleaf hydrangea (Hydrangea macrophylla) ‘Nikko Blue’. Rooted hydrangea cuttings were transplanted in a pine bark/peat mixture. In 1998 and 1999, azoxystrobin at 0.16 g ai/liter and 0.32 g ai/liter, as well as 0.24 g ai/liter myclobutanil (Eagle 40W™) and 0.84 g ai/liter thiophanate methyl (3336 4.5F™), greatly reduced the incidence of powdery mildew compared with the untreated control where 75% of the leaves of were infected by the causal fungus. When applied at 1-, 2-, and 3-week intervals, both rates of azoxystrobin were equally effective in both years in preventing the development of powdery mildew on bigleaf hydrangea. In 1998, all fungicides except for thiophanate methyl protected bigleaf hydrangea from Cercospora leaf spot. In the last two trials, the incidence of powdery mildew increased significantly as the application rate for azoxystrobin decreased from 0.16 to 0.04 g ai/liter and the application interval was lengthened from 1 to 3 weeks. In general, all rates of azoxystrobin applied on a 3-week schedule failed to provide the level of powdery mildew control needed to produce quality bigleaf hydrangea for the florist and landscape market. When applied at 2-week intervals, myclobutanil was equally or more effective in controlling powdery mildew than any rate of azoxystrobin applied on the same schedule. When compared to the untreated controls, significant reductions in the incidence of powdery mildew on bigleaf hydrangea were obtained with weekly applications of paraffinic oil. No symptoms of phytotoxicity were associated with the use of any of the fungicides screened.

Powdery Mildew of Bigleaf Hydrangea: Biology, Control, and Breeding Strategies for Resistance

“…While powdery mildew is not typically considered fatal to bigleaf hydrangea, infection can lead to extensive chlorosis or yellowing of the leaves, premature defoliation, and leaf area and shoot elongation reduction [14]. Disease symptoms tend to be more severe on plants that are in shaded areas with high humidity and limited air movement, such as greenhouses [8,37,38]. Golovinomyces orontii can persist throughout the year in favorable conditions and has the ability to overwinter in the form of fungal hyphae or spores attached to the plant or in plant debris [25,38].…”

“…They are commonly used as inground plants, pot plants, and as cut flowers in the floriculture industry. Bigleaf hydrangea thrive within USDA Hardiness Zones 6 to 9 [8]. These plants are native to China, Japan, and East Asia, can grow three to seven feet in height, and are known for their large, colorful inflorescences [9,10].…”

“…Many plant fungal pathogens can be classified into the phyla Ascomycota and Basidiomycota [18]. One common disease affecting bigleaf hydrangea that falls into the phyla Ascomycota is powdery mildew [5,8]. Powdery mildew symptoms on bigleaf hydrangea are more severe in tightly packed production areas or highly shaded areas.…”

Alteraciones anatómicas e histoquímicas ocasionadas por la oidiosis en hojas de Hydrangea macrophylla (Hydrangeaceae)

“…ocasionadas por nemátodos (French, John, & Williams, 1971;Basterrechea, 2005), virus (Tang, Harper, Wei, & Clover, 2010;Dória, Nozaki, Pavan, Yuki, & Sakate, 2011). Hongos (Hagan, Olive, Stephenson, & Rivas-Davila, 2004;Freire & Mosca, 2009;Windham et al, 2011;Mmbaga, Kim, Mackasmiel, & Li, 2012). Entre los hongos que ocasionan lesiones foliares en H. macropylla se encuentran Cercospora hidrangeae Ellis & Everh (Hagan et al, 2004), Rhizoctonia solani Kühn (Freire & Mosca, 2009), Erysiphe polygoni DC.…”

“…Hongos (Hagan, Olive, Stephenson, & Rivas-Davila, 2004;Freire & Mosca, 2009;Windham et al, 2011;Mmbaga, Kim, Mackasmiel, & Li, 2012). Entre los hongos que ocasionan lesiones foliares en H. macropylla se encuentran Cercospora hidrangeae Ellis & Everh (Hagan et al, 2004), Rhizoctonia solani Kühn (Freire & Mosca, 2009), Erysiphe polygoni DC. (Windham et al, 2011), Alternaria alternata Fr., Colletotrichum gloeosporioides Penz, Phoma exigua Desm, Myrothecium roridum Tode (Mmbaga et al, 2012).…”