–Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Orange rust is a fungal disease that can affect black raspberries and blackberries. We have had several incidences of this disease found in new and established plantings this year.
Initial symptoms of orange rust are spindly shoots with clustered, misshapen, pale green to yellowish leaves. The leaves are covered with bright orange, powdery blisters on the underside. The plant becomes systemically infected and remains so for the rest of its life. The infected plants may give a bushy appearance since many short, upright shoots arise from one bud.
Orange rust does not usually kill plants, but it can significantly reduce vegetative growth and yield. The disease is caused by the fungus Gymnoconia nitens. The orange spores are spread by wind and infect leaves of healthy plants during long periods of leaf wetness provided by rain or dew. Orange rust is favored by relatively low temperatures (50-70°F) and wet conditions. The fungus is systemic and overwinters in the crown and roots of infected plants, leading to the production of new infected canes every year. Without management, the number of infected plants in a field may increase from year to year until most plants are infected.
Orange rust on thornless blackberries
Control starts with purchasing disease free plants. Remove surrounding wild brambles that could harbor the disease. Upon inspection of plants each spring, any infected plants should be dug up and destroyed promptly before rust pustules mature and spores are produced. The location of those plants should be clearly marked, and any new suckers arising from root pieces left in the ground should be removed and sprayed with an approved systemic herbicide. Management practices that improve air circulation, such as thinning out canes within the row, pruning out floricanes immediately after harvest, and effective weed control aid in disease control by reducing build-up of moisture in the planting. Some blackberry cultivars are reported to be resistant to orange rust, but no black raspberry cultivars are known to be resistant.
The best fungicide options are myclobutanil and propiconazole. Fungicides should be applied when the orange pustules are first seen before they burst open and release spores. None of these fungicides will cure an already infected plant but they can protect healthy plants from becoming infected. If a field has a history of orange rust, sprays should be initiated before blisters appear.
This article was adapted from https://www.canr.msu.edu/news/its_that_time_of_year_for_orange_rust_in_brambles
Top and bottom leaf views of orange rust symptoms in a new black raspberry planting.