Evaluation of a Predetermined Fungicide Rotation for Control of Impatiens Downy Mildew in a Texas Landscape

Impatiens ‘Impreza Red’ plants were established in 10 cm pots in a greenhouse for 5 weeks.  During that time, a weekly fungicide rotation including Subdue® Maxx® plus Adorn® (week one), Dithane® plus Rainshield® (week 2), Micora^TM plus Dithane® (week three), Heritage® (week four), and Vital® (week five) was applied to half the plants with the other half untreated.  At the end of this period, field plots were prepared with half of the plots treated with Subdue GR.  Greenhouse treated and untreated plants were then planted in factorial combination in field treated and untreated plots in early May.  Full shade was simulated with 30% saran shade cloth over the field plots.  One week after planting, impatiens plants that were infected with downy mildew and showing strong symptoms were planted in each field plot so that the disease pressure would be uniformly distributed.  Plants were periodically rated for disease using a scale of 0 to 10 with 0 = no disease and 10 = very heavy spores on foliage with heavy leaf drop.  In June, the disease was strongly evident on plants not treated in the greenhouse or field and nonexistent on plants with the other treatment combinations.  Number of plants with crown rot was slightly higher for plants that did not receive any greenhouse treatment, but the differences were small.  In early July, disease was still significant for the greenhouse/field untreated plants and minimal on the other treatments, but the level of disease had decreased greatly from the levels seen in June.  No disease was observed in August on any of the plants.  Crown rot increased from the June to the July counts and was highest for the greenhouse/field untreated plants.  The decrease in IDM incidence during the summer was presumably due to the hotter and drier conditions that prevailed as the summer season advanced.  Greenhouse or field treatments protected plants from downy mildew, though disease pressure decreased as the summer season progressed.