Synthetic and Biorational Fungicides Compared for the Control of Three Foliar Diseases of Flowering Dogwood

Abstract

Abstract The biorational fungicides Neem Concentrate, SunSpray Ultra Fine Oil®, Rhapsody® were compared with the synthetic fungicides Eagle® 40W, Immunox®, Liquid Systemic Fungicide®, and 3336™ 50W for the control of spot anthracnose, Cercospora leaf spot, and powdery mildew on ‘Rubra’ flowering dogwood. In 2003, 2004, and 2005, fungicide treatments were applied at label rates at 1- and/or 2-week intervals over a period that coincided with the onset spot anthracnose and powdery mildew but before the appearance of Cercospora leaf spot. With few exceptions, the synthetic fungicides gave better control of the above diseases than biorational fungicides. Eagle 40W, Immunox, Liquid Systemic Fungicide, and 3336 50W gave equal control of the bract and leaf spot phases of spot anthracnose. Most notable was the carryover of control of the bract spot phase of spot anthracnose into the spring of the following year with the synthetic fungicides. While some control of spot anthracnose was seen in at least one year with the biorational fungicides, none was as effective as the synthetic fungicides. In 2003 and 2004, Neem Concentrate, SunSpray Ultra Fine Oil, and Rhapsody, when applied weekly controlled powdery mildew as effectively as the synthetic fungicides. Efficacy of the biorational fungicides declined under severe powdery mildew pressure in 2005. While Cercospora leaf spot development was slowed by the Eagle 40W, Liquid Systemic Fungicide, and 3336 50W, the biorational fungicides as well as the retail product Immunox also gave some control. An objectionable mottling of the leaves was noted on the flowering dogwood treated with SunSpray Ultra Fine Oil. Control of these diseases with the synthetic or biorational fungicides did not result in an increase in tree height or trunk diameter. While the biorational fungicides demonstrated sufficient efficacy for controlling powdery mildew on flowering dogwood in the landscape, they often failed to provide the level of control of this disease as well as spot anthracnose and Cercospora leaf spot needed to produce quality container- and field-grown nursery stock.