Dips for Control of Propagation Diseases
Scott Clark, Margery Daughtrey, Maria Macksel
Cornell Cooperative Extension
Ithaca, New York
Reprinted from the July 1990 issue of Horticultural News, a Cornell Cooperative Extension publication
Propagation losses are not uncommon in nurseries, even though steps are taken to reduce the potential for infection in the propagation bench. If present, any one of several fungus species may reduce rooting percentage or quality, increasing losses and production costs. In particular, Cylindrocladium scoparium may cause foliage drop, stem cankers, and cutting death during propagation of ericaceous plants. Long Island nurserymen's interest in identifying a quick, effective, and inexpensive method of treating cuttings before sticking to control propagation diseases lead to a series of studies to investigate the effect of cutting dips on rooting and disease control.1
TABLE 1: MATERIALS USED IN THIS STUDY Active Ingredient Product
Current Legal Uses ammonium chloride Green Shield
For control of plant pathogens on inanimate surfaces only at 1/2oz/gal of water; for control of algae on glass and walkways at 1 tsp/gal. benomyl Benlate 50W Foliar, drench and dip treatments of ornamental crops, including dip treatment of growing mix for control of Cylindrodadium and Thielaviopsis on woody ornamentals: immerse plants or cuttings for 10 to 15 min. in a solution of 1 lb./100 gal. ethazole plus thiophanate - methyl Banrot 40W Drench treatment to growing mix for a variety of ornamentals, for the control of Pythium, Phytophthora, Rhizoctonia, Fusarium and Thielaviopsis, at a rate of 6-12 oz./100 gal. applied to 400 sq. ft. copper Phyton 27 Foliar, drench and dip treatments to roses, cyclamen, and geraniums, e.g., a geranium cutting dip for Botrytis control, for 5-10 seconds at a rate of 1 oz./5.2 gal.
Effect on Rooting
In the first study, the objective was to determine the effect of Physan 20 (a disinfectant) at 2, 4, and 8 ml/gal, Phyton 27 (a copper-containing fungicide) at 2, 4, and 8 ml/gal and Banrot 40W (a fungicide registered for drench application to ornamentals) at 1, 2, and 4 oz./100 gal on the rooting of Rhododendron 'Catawbiense Boursault' and R. 'Hershey Red', an azalea. Cuttings taken in August were prepared according to standard nursery practices and then treated for one minute in the various dip solutions prior to sticking into soiless mix.
While neither plant showed any foliar phytotoxicity, there was a difference in rooting of azaleas and rhododendrons. For the azaleas, the root ball ranged from 34 mm in the control cuttings to 43 mm for the high rate of Phyton 27. None of the chemical dip treatments had a negative effect on root ball size. In contrast, the mean root ball diameter for the rhododendron cuttings ranged from 29 mm in the untreated control to 9 mm for the 4 ml Phyton 27 treatment. This was the opposite relationship to that seen with the azalea cuttings, in that all of the rhododendron dip treatments resulted in significantly smaller root balls than were seen for the untreated controls.
The fungicide Benlate 50W (9, 18, and 36 g/gal) and the disinfectant Physan 20 (4, 8, and 16 ml/gal) were assessed as dip treatments for control of C. scoparium during propagation. Cuttings of azalea 'Delaware Valley White' were inoculated with a spore solution of C. scoparium, after which they were immersed in the various concentrations of treatment solution for 5 minutes. Cuttings were then stuck and placed under mist. The azalea cuttings were evaluated for disease symptoms 27 days later.
There were no foliar symptoms of phytotoxicity from the chemical dip treatments (rooting ability was not assessed). In this study, Benlate reduced disease development up to 90%, while Physan 20 gave no control even at the high rate. In the untreated cuttings which were inoculated, symptoms of brown stem cankering and mortality developed in 97% of the cuttings, and the dead stems were often coated with Cylindrocladium sporulation. No stem cankering was observed in the uninoculated controls. The Benlate dips had a highly inhibitory effect on Cylindrocladium infection and symptom development. No Cylindrocladium was recovered from any of the Benlate-treated cuttings. Physan treatments, in contrast, did not significantly reduce disease symptoms, and only the highest rate tested reduced Cylindrocladium recovery from the stems. The high rate of Physan (16 ml/gal) is equivalent to that labeled for inanimate surface disinfestation in the greenhouse.
In another study, Phyton 27 reduced the amount of cankering caused by Cylindrocladium up to 69%, but was not as effective as Benlate in the earlier study.
Certain chemicals may reduce rooting of cuttings of certain plant species. Although in this study, root quality of azalea 'Hershey Red' was not negatively affected by any of the dips tested, Rhododendron 'Catawbiense Boursault' root quality was decreased by all fungicide or disinfectant cutting dip treatments. To avoid phytotoxic effects, nurserymen should determine the effect of dips on rooting on a small number of cuttings before large scale use.
A Benlate dip was effective and a Physan 20 dip ineffective in controlling Cylindrocladium on azalea 'Delaware Valley White.' Phyton 27 also showed some potential for Cylindrocladium repression. Although dips should not be a replacement for good cultural practices, safe and effective dips might reduce the need for soil drenches, which can be expensive and wasteful. Dipping cuttings into a fungicidal solution could be easily incorporated into the cutting preparation procedure. Propagators should determine whether cutting dip treatments are legal by checking the label of the chemical in question. Table 1 outlines the present legal uses for the various chemicals used in this study. In New York State, the intended use of the pesticide must be on the label.
1 These studies were funded by the Long Island Nurserymen's Association Innovative Grant Program, H. Eberhard Memorial Fund.
Editor's Note: Benlate is no longer on the market. According to the U.S. Environmental Protection Agency (EPA) neither Banrot 40W nor Phyton 27 is restricted to commerical use; neither requires a private applicator's license.