The primary concerns for rose growers as fall transitions to winter should be:
- Minimizing the carryover of inoculum—spores and fragments of fungal pathogens which can cause disease the following spring;
- Reducing populations of nematodes and other soil-borne disease pressures to create ideal growing conditions for root flush in January and February; and
- Maintaining rich soil nutrition to support this root flush.
Pathogens holing up in existing infection sites and the development of new spore masses are how rose diseases typically survive the winter. Thus, the potential for disease during the following growing season can be greatly minimized by reducing possible sources of disease inocula. In California and most parts of the USA, the main disease concerns of roses are as follows.
Black Spot of Roses – Diplocarpon rosae
This disease presents as black on rose leaves, with feathery, irregular margins. In later stages a yellow halo develops around the spot, and leaves fall off. Spots may also develop on green stems.
The fungus survives on infected canes and continues to develop fruiting bodies on canes, as well as living and fallen leaves. Leaves and green canes are susceptible to infection. Spores are spread by splashing water.
Powdery Mildew – Sphaerotheca pannosa
Powdery mildew presents with white to gray powdery growth on the upper sides of leaves, green flower parts and stems. Early stages of infection often feature red and raised blister-like areas of affected tissue. The fungus primarily survives as mycelia (threads of fungal tissue) in rudimentary leaves, buds, or inner bud scales. Infection is favored by high humidity and cool weather. The pathogen does not like free moisture. Spores are easily carried by wind.
Rust – Phragmidium mucronatum
Rust develops on lower leaf surfaces and green canes in the form of powdery, orange pustules. Seen from the top side of leaves, these appear as orange or brown spots. In severe cases, defoliation occurs. The fungus overwinters in black pustules on canes and retained leaves. New spores are formed in the spring with increasing availability of free moisture, germinating to reinfect green tissues. Spores are carried by wind currents.
Botrytis Blight – Botrytis cinerea
Botrytis blight results in spotting on petals, with powdery gray spore mats often being formed on affected tissues. The fungus can also attack buds and twigs, causing dieback and twig canker. The fungus has a broad host range, overwintering on dead tissues. Common carryover sites on the plant are cane cankers and blighted twigs. Cool, wet weather favors infection but the fungus has a broad temperature range. Spores are carried by wind currents.
Various Canker Disease – Various fungal pathogens
Canker diseases cause the formation of brown sunken areas on the cane, often gray in color with black, peppery fruiting. Canker fungi overwinter in affected canes. Wounds are a common port of entry for germinating spores, germination being favored by wet weather.
Downy Mildew Diseases – Peronospora spars
Downy mildew produces purplish red to dark brown irregular spots on leaves and all green tissues. Under high moisture conditions, a white, downy mass of spores forms on the undersides of leaves and on other green tissues. The fungus overwinters in stems and buds as dormant mycelia and resting spores (called ‘oospores’) in affected tissues. Spores are carried by wind currents. The pathogen requires high humidity and free moisture for infection.
Dodder – Cuscuta species
This parasitic plant produces yellow to orange strands which entwine stems and leaves of various hosts. The plant, which doesn’t produce chlorophyll, overwinters as a seed. Seed germination coincides with emergence of host plants. Dodder wraps about the host, sends in an infection peg (haustoria), and secures food entirely from the host plant.
How can growers combat fungal plant diseases as the winter period of dormancy starts?
The following steps can assist your efforts at minimizing the effects of the above fungal diseases:
- Prune your rose bush now. Do this to not only shape the bush or tree shape, but also to remove diseased canes and stems which can provide disease inocula places to overwinter until spring. Where cankers or diseased canes and stems can be seen, prune at least 2″ below the zone of obvious disease. This latter practice insures that most of the infection is excised. Make sure that all cuts are made with a sharpened pruning shear. Rough, tearing cuts will promote infection and delay or prevent healing.
- Take all prunings and debris, including dropped leaves and stems, and either discard or place in a compost pile. If used in a compost pile, commence composting now. Do not allow the tissue to sit, as many of the fungal fruiting bodies will continue to mature and produce abundant inocula for spring infections.
- Spray the pruned rose bush with Daconil 2787 fungicide (Ortho) at a dilution of 1 tablespoon per gallon of water. Spray all parts of the rose bush to the point of runoff, making sure to coat all tissues. On exceptionally large cuts made in the cane, paint the cut end with white latex paint mixture, at a rate of 1 tablespoon of Daconil 2787 per pint of paint. Paint the tip of the cut and at least 1” down the cane.
- By the third week in December, spray again with following mixture (at the listed amounts per 2 gallons of water): 5 tablespoons of flowable copper (Ortho), 1 tablespoon of liquid dish soap (e.g. Palmolive), and 2 cups powdered milk. Agitate the mixture thoroughly and spray to coat all surfaces with the copper spray. Spray during the late morning or middle of the day to allow the material to dry on the tissues. If cold and wet weather continues, repeat this spray 20 to 30 days later. Do not use this spray once new growth emerges.
How do you minimize infestations of dodder, the one non-fungal disease listed above?
Over the last few years, isolated dodder infestations have become commonplace—the yellow, vine-like parasitic plant can even often be seen along roadways, enveloping broad-leafed weeds. The impact and devastation to agricultural hosts is made clear in the various common names for dodder infestations: devil’s hair, strangleweed, hellbind, and hailweed.
This parasitic plant, which secures all its nourishment from its host, produces seed. The seed is about the same size and appearance as alfalfa seed. Thus, one of the common avenues of long range spread of the pest is through contamination of commercial seed sources. Animals, such as birds, can also facilitate spread. The first line of defense, then, relies upon sanitation and care in securing clean seed. Warm spring weather in April and May is conducive to dodder seed germination. When detected at an early stage, topical sprays can be used to kill dodder seedlings—such as a mixture of Dacthal herbicide at a rate of 3 ounces per gallon of water, mixed with 2 tablespoons of dish soap.
Many years ago, before the use of diesel or weed oil was banned, growers used to spray with 25% by volume of diesel, with 2 tablespoons of MSMA per gallon of mix. Physical removal and burning is still commonly practiced, and is quite effective. Most importantly, it is imperative that a center of infestation be immediately addressed to curb further spread.
Nematodes and other soil-borne diseases, though not discussed above, commonly parasitize roses and should also be controlled for during the winter season.
Roses are a prime host for a number of plant-parasitic nematodes. Following are but a few of the nematodes we have extracted from around the roots of rose bushes:
- Root-knot nematode
- Ring nematode
- Dagger nematode
- Lesion nematode
- Stubby root nematode
- Stunt nematode
- Spiral nematode
- Pin nematode
- Sheath nematode
In addition to the direct damage nematodes cause to root systems during feeding, nematodes can weaken or predispose the plant to the soil-borne fungal pathogen Verticillium dahliae. The pathogen can move into the vascular tissues of the root system and begin moving up the plant. In the latter stages, plugging of the vascular tissue occurs, and the areas of the rose bush fed by the affected tissue wilts, thus the common name of the disease, Verticillium wilt.
The sustained health of any bush or tree should begin by providing nematode- and disease-free soil. It is especially important to do this before the growth flush of the roots, which occurs in January and February. If not, the newly emerging rootlets will merely provide additional feeding sites for the nematodes.
Maintaining rich soil and foliar nutrition is crucial in hardening roses against disease during the winter season.
Roses, like most other agronomic plants, respond negatively to sudden excesses of nitrogen. They also do best with balanced nutrition. I recommend following this general guideline:
- Always place about 50% of your yearly fertilizer down in late Sept or early October. Wherever possible, use a calcium-containing source of nitrogen or supplement with a triple mix, such as 12-12-12 with calcium.
- Begin incrementing the remaining 50% with the first flush of growth in April by spreading 10% at that time. Increment the remaining 40% during the months of May, June, July, and August.
- If you have problems with aphids or white flies, use a fertilizer which contains a systemic pesticide. Be sure to lay this down as your first and third incrementing fertilization in April and June.
- Before and during heatwaves, spray with a triple mix foliar. For example, a mixture of 2 ounces of 9-15-30 (or comparable), 2 ounces of liquid calcium (such as calcium glucoheptonate), and 1 tablespoon of dish soap per 3 gallons of water. Spray in a fine mist and avoid drenching. Spring and summer time sprays are best conducted in the early morning or late evening. Remember that the longer the nutrient spray stays in a liquid droplet, the more absorption into the tissues will occur.
- If powdery mildew is a problem, include 2 tablespoons of Daconil 2827 into the above nutrient spray. Also, if rose beetles, aphids or other insects pose a problem, the above nutrient spray is compatible with most insecticides.