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Plant Disease Control

Powdery Mildew looks like white fuzzy powder that accumulates on leaves and stems predominantly in spring, and again to a lesser degree in fall. It is actually a fungus that is spread by millions of microscopic spores. It imbeds itself into tender new growth and feeds on the sap of the plant. By the time the naked eye can see the white ‘powder,’ it has already invaded the plant tissue and is feeding and reproducing at a rapid pace. As it spreads itself on the surface, it eventually kills the cells of the plant leaf, leaving the leaf rippled and curled.

Mildew spores are everywhere in the garden – in the air, the soil, on debris and on plant surfaces – ready to sprout when the environment is just right. Warm days (50-80F) and cool nights with elevated humidity and resultant dew provide ideal conditions. Though humidity promotes fungal growth, it grows on DRY plant surfaces, unlike blackspot which requires immersion in water for about seven hours in order for infection to take place. Tender new growth needs a chance to ‘harden’ and develop its waxy coating that provides somewhat of a barrier to fungal growth.

Therefore, the rosarian must provide protection for new spring growth on a weekly basis. Controlling mildew doesn’t have to mean spraying the planet into oblivion. It includes plant genetics, cultural practices and something as simple as WATER. GENETICS: While rose hybridizers are chastised for breeding OUT fragrance, what they are trying to accomplish is breeding IN disease resistance. For scientific reasons beyond explanation here, rose genes don’t contain both features – it’s one or the other. Hence, you can expect either fragrant roses ith little disease resistance, or clean plants with little fragrance.

Plants with glossy or waxy leaves are less susceptible to mildew, as the leaf surface is harder for spores to penetrate. Rugosas naturally possess a high degree of disease and pest resistance. Where mildew is a constant problem, the choice in plantings can help prevent the need for extensive maintenance. CULTURAL PRACTICE: Planting bushes with sufficient space between them and away from walls and fences will provide good air circulation which reduces the chances for mildew. The annual pruning event plays a major role in disease prevention.

Stripping leaves from the bush at pruning time, and cleaning up debris in the garden contribute to a cleaner environment. Dormant spraying will at least wipe out last year’s spores, leaving only this year’s to contend with. Keeping the centers of the bush open during the growing season will aid air circulation. Avoid the use of other plant materials with high mildew susceptibility, such as euonymus and tuberous begonias. Apply a thick layer of mulch in early spring to cover spores in the soil that may have wintered over. WATER is perhaps the most misconceived element surrounding powdery mildew.

Many gardeners still subscribe to the belief that you should NEVER get rose foliage wet. On the contrary, a high-pressure spray of water will remove mildew spores that haven’t imbedded themselves yet, and prevent them from germinating. Higher incidence of mildew during periods of rain is caused by the moisture in the air and soil – increasing the humidity that promotes mildew – not by water on the leaves. Similarly, watering early in the day will allow the soil surface to dry out a bit before the cool night temperatures arrive, reducing humidity from moist soil. PREVENTION IS THE ONLY CURE

Once powdery mildew is apparent to the eye, it can’t be eradicated. It simply must be prevented. Prevention is achieved by coating the plant tissue with something that provides a barrier to prevent fungus from gaining a foothold and invading the plant tissue. Growth is so rapid in spring that the leaves unfolding THIS week won’t be protected by what you sprayed LAST week. This is the reason you find application schedules of every 7-10 days on most fungicides, and the reason you must follow that schedule. The choice of what the SOMEthing is that you choose to spray is widening.

Fungicides are the most idely used because they are chemically formulated to specifically combat fungus diseases. Recent reports of non- toxic, environmentally-friendly products such as baking soda and anti-transpirants are proving very encouraging also. FUNGICIDES are any of a number of chemicals labeled to combat powdery mildew, and do so by interfering with its metabolic life process, rendering it unable to grow and spread. Although they must be in place on the plant before the spores arrive, they do have systemic action – meaning they move into the plant tissue – providing a residual effect for a short period.

Fungicides are available in many forms – liquids (mix readily with water), emulsifiable concentrates (a thicker, usually milky substance), wettable powders (require thorough mixing prior to application). Each has its own properties, all are effective. Most, however, have a medium-to-high degree of toxicity to humans. Extreme caution should be used to cover eyes, skin and hair, and use a painter’s mask or respirator during application. They are mixed at various rates, usually 1 tablespoon per gallon of water, and require application every 7-10 days.

BAKING SODA: “New research shows that simple baking soda is a powerful weapon against ungus-caused rose diseases”, wrote Kristi Clark in her September 1992 American Rose Magazine article. In a world that is becoming increasingly aware of environmental concerns, more attention is being paid to finding alternative measures to widespread chemical use. Sodium bicarbonate (grocery-variety baking soda) was tested originally to determine its effectiveness in preventing blackspot. During the experiments, it was noticed that no powdery mildew was found on any of the test roses.

Controlled experiments were conducted for some three years, using sodium bicarbonate or otassium bicarbonate in various combinations with insecticidal soap, Sunspray ultra-fine spray oil, or only water. The result: both diseases were subdued by a weekly spraying of either sodium or potassium bicarbonate at 3 teaspoons per gallon of water, combined with Sunspray at 2 tablespoons per gallon of water. The bicarbonates eliminated the fungi, but addition of the Sunspray provided a spreader-sticker action that increased its performance. Sunspray is available commercially as Safer Sunspray.

As Clark cautioned, do not attempt higher concentrations of the solutions, as leaf burn may result. Rain or overhead watering may wash the solution off, reducing its effect. ANTI-TRANSPIRANTS are another group of substances that hold promise as a non-toxic method of controlling powdery mildew (as well as pests). Anti-transpirants are emulsions and acrylic polymers that were developed to form an impermeable film on plant surfaces to substantially reduce moisture loss. Several brands are available; look for a white liquid, about the consistancy of milk.

They are widely used on cut Christmas trees to retard drying and needle drop, and on plants to provide protection from drought, heat, wind and transplant shock. Since the thin film prevents transpiration of moisture – both in and out of the leaf – it makes sense that it would also prevent fungus spores from permeating the leaf surface. Some rosarians have used antitranspirants in combination with fungicides, and feel the combination works better than fungicide alone. Others have used it entirely alone, and find that it works very well all by itself.

Packaging directs us to water plants well and allow them time to take up the water before spraying. Since anti-transpirants are NOT yet labeled for disease protection, there is no accepted formula for application. They come in various concentrations that would require more or less dilution – anywhere from 1 tablespoon to 1/2 cup per gallon of water. Again, frequency is not addressed … once a week … once a month? At this stage it’s sort of experimental. If a residue is left on the foliage (objectionable to you as an exhibitor) then reduce the ratio.

Whether we choose the fungicide method or the non-toxic approach to controlling powdery mildew probably depends upon the degree of severity we encounter on a regular basis. Regardless of the product selected, it must be used on a regular basis in the proper dilution to prevent fungal growth ithout damaging plant tissue. What is Blackspot? Symptoms Blackspot is a plant disease caused by a fungus (Diplocarpon rosae) that is generally damaging and usually a source of major problems. Blackspot looks like circular black spots with irregular edges on the top side of the leaves.

The tissue around the spots or the entire leaf may turn yellow and the infected leaf may drop off. Plants with a severe case may lose all of their leaves if not treated. Flower production is often at a minimum and the quality of bloom suffers badly. Biology High humidity is one factor that helps the spores to germinate. The spores germinate in 9-18 days on a moist leaf at 70-80F temperatures. The spores can be spread by splashing water and by the Rosarians themselves. The spores are wind-borne only in water drops.

The spores can be spread on clothing, tools or even your hands, but the way it is spread most often is by infected leaves that have wintered over in the rose bed. Control Blackspot can be satisfactorily controlled by spraying with a good fungicide every seven to ten days (read the label and follow the directions). There are also a number of measures that should be taken to keep from getting and/or controlling the disease. Avoid watering in a way that splashes water up on the leaves and avoid watering late in the evening with a hose or sprayer.

Make sure to clean up the beds completely of all leaves or stems to help keep the disease from wintering over. Always have good ventilation through the plant and good soil drainage. Apply fungicides after a rain to keep down spores. Put the plants on a spray schedule and spray with a fungicide that gives good control, such as, Manzate, Maneb, Daconil and Lime-Sulfur compounds. There are also organic methods of controlling Blackspot. Baking soda has been tried as a cure and s a preventative measure. It was found that using baking soda and spray oil mixed with water as a spray can damage roses if it is not mixed in the proper proportions.

It was also found that baking soda gave only moderate control of Blackspot, but appeared to be effective as a preventative. There is a new product coming on the market that has been used by our local Rose Society that does show promise. This product is derived from the Neem tree. It is called “Rose Defense” by The Green Light Co. One other way to prevent Blackspot is to plant roses that are disease resistant. There are some roses that have some resistance built into their genes. But remember, they are Resistant not Immune. They still need to be sprayed on a regular schedule.

Roses should be kept on a regular spray schedule regardless of which method is used. Remember, prevention is the key to controlling Blackspot. Rose Mosaic Virus Disease by Malcolm M. Manners, Lakeland, FL Many of you know that the primary reason we grow roses at Florida Southern College is our involvement in indexing and heat-treating roses for rose mosaic disease. While we have had articles bout the subject in numerous other publications, over the past decade, I’ve not mentioned the subject in The Cherokee Rose, nor has there been any extensive discussion of the subject at any of our meetings.

Yet it is a subject I believe to be quite important, particularly in that a grower, through ignorance of the problem, could introduce a viral infection to an antique rose which may have survived hundreds of years without the disease. A few simple precautions could have prevented the infection. Also, some old rose nurseries are notorious for shipping virus-infected plants, while others ave made a great effort to provide virus-free bushes. I certainly commend (and recommend) the latter group.

The following is an updated version of a paper I presented to the Florida State Horticultural Society, in 1985: The Citrus Institute of Florida Southern College initiated a program to rid infected rose plants of rose mosaic (RM) disease in 1984. This paper will describe the disease, its effects on rose plants and their culture, and the heat therapy program at Florida Southern College. Rose mosaic is a disease caused by a virus complex infecting cultivated roses (Rosa spp. and hybrids). Cochran 3 reported that by 1970, most of the garden roses in the United States were infected.

Since then, heat therapy programs have been initiated at the Oregon State University and the University of California at Davis, as well as by Bear Creek (parent company of Jackson & Perkins Roses and Armstrong Roses). The Oregon State program is now nearly defunct. Some commercial rose nurseries have made use of those programs and now offer virus-free plants for sale. However, many nurseries have not made any attempt to provide healthy plants, and a large percentage of the roses grown and sold in Florida are infected.

Florida nurseries using Fortuniana as a rootstock are at a particular disadvantage, since scion-source plants of new cultivars are received from a single source, usually on Dr. Huey rootstock, from California. If these original plants are infected, then all plants subsequently produced on Fortuniana rootstock will be infected. In recent years virtually all new cultivars, including the All America Rose Selections (AARS) winners, have been infected with RM when received by the Florida nurserymen (personal communication from several nurserymen. Diagnosed by leaf symptoms.

The disease also may be spread to other cultivars through the use of infected rootstock. No source of indexed virus-free Fortuniana plants has been available until recently, although some propagators have been quite conscientious about selecting their rootstock cuttings only from plants which have never shown symptoms of RM. Since RM is not fatal to the plant and often has no obvious detrimental effect on a rose, nurserymen and rosarians tend to be unconcerned about the problem. When leaf symptoms appear on a plant, the affected branch is pruned off, temporarily ridding the plant of its symptoms.

If (as many growers believe) the only effect of RM were an occasional chlorotic or disfigured leaf, there would be little cause for concern about the disease. However, RM has been shown to cause flower distortion 2,3,4,8, reduced flower production 3,4,6,8,9, reduced flower size 8,9, reduced stem caliper at the graft union 8,9, reduced vigor 2,3,7,8,9, early autumn leaf drop 8, lower bush survival rates 6, increased susceptibility to cold injury 6, and more difficult establishment after transplanting 8. The symptoms are highly variable among rose cultivars and are strongly influenced by weather and rowing conditions.

Infected plants may appear to be quite healthy for much of the year, and any symptoms which do appear may be attributed to other causes, such as spray burn, nutrient deficiencies, high temperature, or poor horticultural practices. It has been suggested that the “deterioration” which often occurs in rose cultivars several years after their introduction may be a result of virus infection 1.

Other leaves may show a bright yellow or white “oak leaf” or “mosaic” pattern . A very faint “watermark” chlorosis is common on the leaves of some cultivars . These symptoms often fade as the leaf ages and may disappear completely. The chlorotic patterns associated with RM usually do not closely resemble any mineral nutrient deficiency or herbicide toxicity pattern and are reasonably reliable for diagnosing RM. The absence of any obvious symptoms is normal, and is no guarantee of freedom rom RM; some infected cultivars seldom show symptoms, but their performance may be impaired.

The Heat Therapy Program at Florida Southern College Florida Southern College’s heat therapy program was initiated with the following goals: To produce rootstock plants adapted to rose culture in Florida that are known to be free of RM, particularly Fortuniana and Fun Jwan Lo . To rid commonly grown scion cultivars (including old garden rose cultivars) of RM. To provide propagating material of rootstock and scion cultivars to nurseries interested in cooperating with the program, thus enabling Florida residents to purchase isease-free plants on desirable rootstocks.

To maintain a RM-free garden for the preservation of healthy germplasm of the treated cultivars. The heat therapy procedures are similar to those employed by the programs at the Oregon State University and the University of California at Davis. Infected scionwood is budded or grafted to Fortuniana rootstock and grown to a 2-gallon size plant. The potted plant is placed in a controlled-environment chamber, where the temperature is held at a constant 38 C (100 F) for 21-35 days. The heat treatment does not cure the plant, but RM-free aterial can be obtained as follows: Axillary buds from the treated plant are budded onto RM-free rootstocks.

Most of the axillary buds on the heat-treated plant will be free of RM. Once the new budlings are growing, they must be tested to insure freedom from RM, a process known as “indexing. ” We use three indexing methods: Mme. Butterfly — Buds from the plant to be tested are budded to established plants of virus-free Mme Butterfly an older Hybrid Tea which shows brilliant mosaic symptoms when first infected. This is usually done in the autumn. The plant is allowed to grow a new flush of Mme. Butterfly leaves during the spring, and those leaves are observed for symptoms.

Shirofugen — Buds from the plant to be tested are budded to branches of Shirofugen a Japanese flowering cherry tree. Roses and cherries are not graft-compatible, so the graft always dies. If the bud was not infected, the cherry branch heals over, cleanly. But if the rose bud contained mosaic virus, the virus will be transferred to the cherry branch, which will react by producing a sticky, gummy oozing sap, and the area around the graft union will die. Cherry trees don’t grow well in Central Florida, so we contract with he University of California to do this test for us. We ship them budwood to be tested, in June.

ELISA — Enzyme-Linked Immunosorbent Assay is a laboratory method, using rabbit antibodies. It is a quick (less than one day) laboratory test, and not only tells whether any virus is present, but can often determine exactly which virus, and sometimes even which specific strain of a virus, is present. We contract with the Washington State University, to do this test, sending them leaf samples in cool weather. The program at Florida Southern College is now nearly 10 years old. We have heat-treated and/or ndexed hundreds of varieties, and now maintain more than 350 virus-free scion varieties, including around 200 old garden roses.

We also have virus-free rootstocks, including Fortuniana Fun Jwan Lo and Dr. Huey. Mosaic-free plant material is available to commercial nurseries for propagation, and it is through our cooperating nurseries that mosaic-free plants are available to the public. Summary and Conclusions Rose mosaic disease currently infects a large percentage of the roses grown in Florida, and throughout the United States. While hobbyist growers and most nurseries lack the facilities to rid lants of the disease, cultivars can be freed of RM by a simple heat treatment program.

Florida Southern College is engaged in such a program, and offers virus-free material to commercial nurseries, to the extent that time and facilities will permit. Since RM is believed never to spread by natural means, there is no legitimate excuse for its continued existence in American rose nurseries and gardens. While RM is not deadly or otherwise devastating to a rose bush, improved growth and more flowers of higher quality may be expected from disease-free plants, so it is to a grower’s advantage to seek out plants known to be free of the disease. You can’t cure it in your garden, but it is not going to spread from bush to bush.

So there is no great need to dig up and destroy an infected bush. However, if you can find a virus-free plant of the same variety, you might want to consider replacing the bush, to gain more vigor and greater flower production. 2. If you do your own budding or grafting, remember that those procedures spread the disease, so try to use virus-free scion wood and virus-free rootstocks. If you root cuttings of Fortuniana that sprouted out from the base of a grafted bush, remember that those cuttings will ontain the virus if the original bush was infected.

Also, any scions collected from an infected bush will produce infected plants, when propagated. 3. Remember that a complete lack of symptoms (i. e. , a healthy looking bush) is the normal situation for an infected plant. Just because a plant appears to be healthy, even for several years, is no guarantee that it is indeed virus-free. Only indexing can tell you for sure. 4. One of the major reasons so many nurseries are “cleaning up” their stock, in recent years, is customer demand. Please support and commend nurseries that produce clean plants.

Encourage nurseries who don’t, to begin growing virus-free roses. If they know it is important to you, the customer, they will likely respond favorably. While I am not aware of any nursery which sells only virus-free plants, most of the nursery-members of the CFHRS do grow at least some clean varieties, and will gladly tell you, if you ask, which of their stock is clean. It will be quite a long time until all of the commercially propagated heritage roses can be cleaned up, but we’ve made a good start. Here’s a partial listing of older roses available from our program, through retail nurseries:

Insecticide chemicals have been linked to childhood immune disorders, nervous system problems and hyperactivity. Chemicals commonly found in insecticides-like PCB’s and DDT- can cause negative estrogen-like effects in some women, contributing to breast, ovary and uterus cancer. Home pesticide users may use an average of up to six times more pesticide per acre than farmers. Insecticide use has increased ten-fold since 1940, but insect induced crop losses doubled to more than 13 percent. 25-50 percent of air sprayed pesticide does not hit the field and drifts into the environment, contaminating soil, water, and air.

Pesticide residues on fresh produce can be reduced by thorough washings with water, removing outer leaves, peeling and cooking. However, not all residues can be removed, especially residue from pesticides that enter fruits and vegetables through the soil. Pesticide chemicals remain in the environment long after they are no longer used-DDT, chlordane and heptachlor can linger in the soil for more than 20 years. Consuming organically grown foods and using alternative pesticide control methods can effectively decrease chemical contamination of humans, animals and the environment.

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