Black rot of stentlings of roses: A disease caused by various soil fungi

Abstract

Stenting is a technique for the production of genetically uniform plant material of roses. Stentlings, the young plants obtained by a combination of cutting and grafting, are susceptible to black rot of the rootstock. The first symptom is a dark discolouration of the rootless base of the rootstock. The disease is most harmful to newly made stentlings. It leads to complete deterioration of the stentling and can cause unacceptable losses. Leaf-bearing cuttings and older stentlings of which the rootstock and leaf-bearing scion have grown together were hardly affected by black rot, suggesting that assimilation products are necessary to confer resistance to the tissue. Treatment of the rootstocks with indolyl-butyric acid (IBA) in order to accelerate root formation promoted incidence of black rot. The increased incidence was not due to stimulation of mycelial growth of the pathogen by IBA. It is suggested that it may be attributed to an increased sensitivity of the rootstock caused by depletion of nutrients for expression of resistance. Several fungi were isolated from the diseased rootstocks. Isolates of the most common species were tested for their pathogenicity. Black rot was caused by 12 out of the 14 species tested. The most aggressive pathogens were Ceratobasidium sp., Coniothyrium fuckelii, Fusarium oxysporum, F. redolens, F. solani and Pythium ultimum. Moderately pathogenic were Alternaria alternata, Cylindrocladium parvum, Ophiostoma piceae and Gliocladium roseum. None of the three fungicides tested controlled the disease completely. Captafol, which has recently been withdrawn from the market, was by far the most effective one. Benomyl and, to a lesser extent, captan were effective during the first 5 days of incubation.