Disease-Resistant Knockout Rose Susceptible to Rose Rosette Disease
Remember when Knockout rose was the answer to our rose disease problems? Suddenly, they were everywhere. I, too, am guilty of mass plantings in both residential and commercial settings. Then came rose rosette disease; Knockout was susceptible.
During the past 10 years or so, rose growers have struggled with this previously mysterious affliction. The disease was first described in the 1930’s but it began to spread through our area in the 1980’s. With the introduction of the Knockout series, roses became popular again. Consequently, with more roses came more extensive disease spread.
Rose rosette disease was first diagnosed on Knockout rose in Kentucky in 2009. Since then, the UK Plant Disease Diagnostic Lab has witnessed a rapid increase in incidence, especially with the disease-resistant Knockout. This year seems consistent with that trend.
Symptoms: Disease symptoms vary with rose cultivar, but combinations of all or some of the most distinct symptoms are used for diagnosis.
· Stem bunching or clustering, witch’s broom
· Elongated and/or thickened canes
· Bright red leaves and stems
· Excessive thorniness, small red or brown thorns
· Distorted or aborted flowers
· Under-developed or narrow leaves and/ or distorted canes
· Dead or dying canes, yellow or brown foliage, dwarfing or stunting
Causal Agent: Causal agent of rose rosette disease is a virus classified in the newly described genus Emaravirus. It is systemic and occupies all tissue with a plant, although only some plant parts may be symptomatic. This pathogen is not spread by pruners or other mechanical means like some common viruses, but it is readily transferred onto rootstocks through grafts. Due to high demand of Knockout roses, mass production has led to spread of diseaed roses throughout industry.
In the landscape or garden center, disease is spread by a small mite, the rose leaf curl mite that resides in axillary buds. Mites are transported on insects or by wind currents for up to 100 yards.