This article is to share some information regarding orange rust in blackberries on the Central Coast. Apparently, this fungus was detected last year and it continues to spread. Previously only seen in some local plantings of Chester blackberries it has now been found on several occasions in proprietary blackberry plantings. Orange rust is a tough disease to deal with, so it is worth being able to identify and knowing what steps one needs to take to mitigate its spread.
Orange rust is caused by two fungi, Arthuriomyces and Gymnoconia which are distinguished by the shape of their spores and life cycle length. Their growth is favored by cooler temperatures and high humidity. While it is not common that orange rust infected plants die outright, their ability to produce fruit is severely compromised.
As readers can clearly see in the photos below, orange rust is hard to miss in the field. From further away, infected canes have a spindly appearance and on approaching one will see the upper leaf margins of both primocane and floricane framed with the distinctive orange of the fungal infection on the underside of the leaves.
Of all the rust fungi that we deal with in caneberries on the Central Coast, orange rust is unique in that it grows systemically in the plant, meaning that the most important management tool for growers dealing with an infestation of orange is a shovel. There is no effective fungicide for orange rust. Infected plants should be removed entirely, meaning all canes, leaves and the roots. This is best done before the pathogen spores are ready to be spread by rain and wind in mid-April through May.
Author - County Director, Santa Cruz County and Farm Advisor, Strawberries & Caneberries
Orange rust on blackberry- note relative spindliness of infected leaves compared to those not infected in background
Underside of leaf infected with orange rust