Extension Educator, Horticulture
Which do you want to hear first – the good news or the bad news? In the last few years we have seen a disease in multiflora roses called rose rosette. This may seem like good news since multiflora rose is a real pest. It was designated as an exotic weed along with Japanese honeysuckle and purple loosestrife under the Illinois Exotic Weed Act with good reason. It invades native areas and chokes out native plant species. The Exotic Weed Act states it is unlawful for any person or agency to buy, sell, offer for sale, distribute or plant seeds, plants or plant parts of exotic weeds.
A disease killing out multiflora rose sounds like good natural control of a pest. However, cultivated roses may also become infected with rose rosette. Climbers, hybrid teas, floribundas, miniatures and a number of old fashioned roses have been reported as hosts for this disease.
Rose rosette is believed to be caused by a virus-like organism and it is thought to be spread by a tiny mite. A mite so tiny 20 would fit on the head of a pin. As the mite feeds it transmits the disease agent. Of concern to rose growers, rose rosette can also be spread by grafting.
Rose rosette is fairly easy to diagnose since symptoms are fairly distinct. According to Nancy Pataky of the University of Illinois Plant Clinic, the new growth of leaves and stems of infected plants appears a deep red. Actually kind of a pretty color like autumn leaves. The leaves may show crinkling and distortion or a color combination of greens, yellows and reds. Rapid stem elongation can be an early symptom in garden roses. Later the plant produces numerous small succulent shoots growing in different directions giving the stem a "witches' broom" effect. The shoots are usually deep red and much thicker than the original canes. The shoots also have many more thorns. The stems may seem almost "furry" with thorns. Some plants may show a loss of symptoms for a few months but the symptoms will reappear. The plants usually die within 22 months of infection.
Control of rose rosette in cultivated roses takes courage. Infected plants should be removed as soon as rose rosette is identified or confirmed. Yes, completely removed. Rose rosette is not picky about attacking that $50 rose you just bought. Dig up and remove all plant parts including roots. This material should not be used in the compost pile.
According to Pataky controlling rose rosette through mite control has not been shown to work with any consistency. Research suggests that the critical mite transmission occurs in May and June. If miticide such as dicofol sold as Kelthane are used as a control option, concentrate efforts in those months next year.