Rose Pests and Spraying
(for Northeastern US gardeners)
by Mark Huss
This article is primarily about spraying roses to prevent disease pests and deter insect pests. These sprays are generically called "pesticides." I first discuss the safety issues that anyone who sprays should be aware of. I then briefly discuss the diseases and insects which are our biggest problems here in the northeast (I hope to later expand this to be less parochial). Next, I describe fungicides and list several good for roses. Finally, I discuss some eco-friendly soultions and alternative products.
I am an avid rose gardener who lives in southeastern Pennsylvania, USA. The information contained here is based on my experiences in my garden as well as a lot of reading. I spray fungicides regularly in season and insecticides little to not at all, strictly on an as-needed basis.
Note: The specific pesticide product descriptions below and suppliers mentioned are for informational purposes only and do not indicate an endorsement by the author or society. Many of the products discussed are not available in your local store and will need to be ordered from mail-order companies such as Rosemania, Muncy's Rose Emporium, or Perfect Products.
Sprays: How toxic is toxic?
Signal Words and LD50s
One important aspect of understanding chemical safety is recognizing the differences in the toxic potential of pesticides. Pesticides are grouped into four subgroups based on their ability to cause lethality (death) or to damage the eyes or skin. The standard measure of lethality is known as the lethal dose 50 (LD50). The LD50 value for a pesticide is the dose (grams/kilograms of body weight) of the pesticide that will kill 50% of an exposed population(of mice or rats or other test animals). The lower the LD50 value, the more toxic the pesticide.
The most toxic pesticides carry the signal word DANGER on the label, are listed as highly toxic and are classified by the Environmental Protection Agency as Category I pesticides. Based on lethality information, it would only take ingestion of a few drops of a liquid or about 3.5 grams (remember that there are 454 grams per pound) of a solid to kill a 150 lb man. Category I pesticides with this lethality potential can only be applied by licensed applicators, and are not available for general use by the home gardener. However, some pesticides available to the home gardener are listed as Category I pesticides based on their ability to produce severe skin and/or eye damage. These pesticides include: Orthenex, Vendex, Isotox, Mavrik 2E, Captan dust or powder, Triforine EC, Lime Sulfur and Copper Sulfate (99%+).
The next most toxic pesticides carry the signal word WARNING on the label, are listed as moderately toxic and are classified as Category II pesticides. It would take ingestion of 1 teaspoon to 1 ounce of a liquid or between 3.5 grams to 35 grams of a solid Category II pesticide to kill a 150 lb man. Pesticides in this category include: Cygon 2E, Daconil 2787, Diazinon 25%, Metalaxyl (Subdue 2E or Ridomil), Rubigan (some preparations), Permethrin and Nicotine.
Members of the third most toxic group of pesticides carry the signal word CAUTION on the label, are listed as slightly toxic and are classified as Category III pesticides. These pesticides require ingestion of between 1 ounce to a pint of a liquid or 35 to 350 grams of a solid to be lethal to a 150 lb man. Pesticides in this category include: Avid, Carbaryl, Mavrik Aqua Flow, Captan 50W, Immunox, Maneb, Mancozeb, Manzate, Rubigan (WP, EC), Sentinel (cyproconazole), Pyrethrin, Acephate (Orthene) and Malathion.
The least toxic group of pesticides also has the signal word CAUTION on the label, but is listed as toxic and classified as Category IV pesticides. These pesticides require amounts greater than a pint for a liquid or greater than 350 grams for a solid to cause death in a 150 lb man. Pesticides in this group include: Benomyl, Copper Sulfate solutions, Diatomaceous Earth, Sulfur WP, Neem Oil and Safer Soap.
It is important to note that none of the categories are listed as nontoxic. That means that even though a pesticide has only a small likelihood of being harmful, caution should always be used when handling these chemicals, particularly in concentrated form.
Clothing - wear long sleeves and pants (no shorts), rubber shoes, rubber, latex or nitryl (NOT cloth or leather) gloves
Goggles - Use the chem-lab variety (with covered vents) not "sunglasses."
Respiriator - Use a "gas mask" (cartridge type), not a dust mask - remember, if you can smell it, it’s getting in your lungs!
Before you spray
When you spray
After you spray
Fungal diseases are probably the number one problem for rose growers. Fungi are tiny organisms that parasitize the host plant and spread via microscopic spores. Some fungi disfigure the plant but some can defoliate and kill a susceptible rose bush. Roses vary widely in their susceptibility to fungal infection. Some (for example, rugosa species and near-hybrids) almost never are bothered, others (most fragrant hybrid teas, for example) are "fungal magnets."
It is much easier to prevent a severe fungal infestation than to control it once it gets established. Cultural practices that discourage fungus include:
You can avoid a lot of trouble by selecting varieties that are known to be disease-resistant. Note: Ask a local rosarian about this, do not trust the catalog descriptions! Of course, many people want to grow fragrant Hybrid Teas (and other susceptible varieties). It is for those folks this article is intended.
Listed below are the most common fungal diseases in our (northeastern US) area. Most of the fungicides listed below are labeled for most or all of these diseases.
Black Spot Symptoms
Key Black Spot Facts
Black Spot is probably the single biggest problem for those of us in the humid Eastern US. Other problems come and go, but black spot can defoliate and eventually kill a susceptible rose bush. Rose varieties vary widely in their susceptibility to black spot, with Hybrid Tea, Grandiflora, and Floribunda being in general the most susceptable. I have also noticed that fragrant roses tend to be susceptible.
Black spot (generally classified as Diplocarbon Rosae) is a pathogenic and contagious fungus, which infects and penetrates rose leaves and produces a multitide of spores which can then infect other leaves. Spores are most often spread by the splashing of raindrops or overhead watering. It is incurable in the sense that once a leaf is infected, there is no cure for that leaf, and it is best removed as soon as possible to prevent it infecting other leaves. Although an infestation of black spot cannot be cured, it can be prevented.
One way to avoid black spot is to only plant rose varieties which are resistant to it. Examples of highly resistant varieties include Knock Out, Carefree Beauty and Cherry Meidiland. Most Rugosa hybrids (e.g., Hansa) are also highly resistant. I grow all of these and they never get sprayed.
If you want to grow roses that look like the florist rose (high-centered Hybrid Tea) in this area, you'll have to do some spraying.
The simplest and safest spray is water. If you regularly wash the foliage with a spray of water to rinse off the spores before they germinate, you can greatly reduce the rate of infection. (Do this early in the day so that the foliage can dry relatively quickly).
There are also a number of spraying materials you can buy which are formulated to kill or prevent fungus, including black spot. The advantage of a commercial spray is that you only have to apply it every week or two (the water spray must be done every couple of days.). The disadvantage is they all cost money, and are toxic to some degree.
Key Powdery Mildew Facts
Powdery Mildew is another contagious fungal infestation. It rarely is a serious problem in this area,as the conditions necessary for it to thrive (warm days and cool nights) only occur briefly here in the spring and fall. However, it can ruin the foliage of a rose for exhibiting, so it must be dealt with in that situation. Plus, it is just plain ugly!.
Key Downy Mildew Facts
Downy Mildew is another contagious fungal infestation. It rarely is a serious problem in this area, as the conditions necessary for it to thrive (warm days and cool nights) only occur briefly here in the spring and fall. However, it can rapidly defoliate susceptible rose cultivars, so it must be dealt with if you grow any of these.
Insect and Mite Problems
Conventional chemical controls are available to control all of the bugs listed below. In these thumbnail descriptions, I discuss alternative methods of control when available.
Aphids are small green to brown insects which infest the tips of new growth.
They secrete a sticky sugary substance that ants love. Except in severe
cases, they are not a threat to the plant. The best way to get rid of aphids
is to knock them off with a jet of water.
Thrips are very tiny insects which use their scraping mouthparts on rose
flower petals, which creates brown spots on the petals. This is the only
problem they cause, so they are primarily a problem for exhibitors and
other perfectionists. They (annoyingly) seem to prefer lighter colored
petals on which the damage is most obvious.
The rose midge is a tiny mosquito-like insect about 1 mm long. It lays eggs in the tips of new rose growth, including flower buds. These hatch into tiny maggots that feed on the rose. The maggots then usually drop off the rose to the ground to pupate, after which the new growth turns black and dies. These "burned tips" are the warning flag that the midge has invaded.
Unlike the other bugs discussed so far, this one can be serious, because in
the extreme case, you'll end up with a garden full of rose leaves but no
flowers! There are chemical controls available, but I'm not aware of any
"envio-friendly" means for controlling this pest.
This heading encompasses a number of different insects, all of which cause
damage by eating the leaves and sometimes the flower buds. This is rarely
dangerous to the plant, but it can be disheartening to see plump buds with
holes munched into them. A eco-friendly solution for many of these is
called BT, which is a bacteria that only attacks caterpillars that munch on
the plant you've sprayed.
This heading also encompasses a number of different insects, all of which can cause serious damage to foliage and flower. The most infamous in the northeast is probably the Japanese Beetle, but the Cucumber Beetle is also a major past in these parts, and also the sneaky Chafer Beetles -- these brown bugs only come out at night, and can only be "caught in the act" by flashlight.
Neem and botanical pyrithrins are somewhat effective on the leaf chompers.
The best eco-solution is to get a cup of soapy water and go around knocking
the beetles off your roses into the cup. Japanese Beetles tend to drop
while taking flight which makes them easier to catch. My young nephews enjoy
going beetle-hunting for chafers with flashlight in hand.
These nasty critters not only attack your roses but many other plants (both inside the house and in the yard) as well. They are microscopic mites which suck the juices out of the leaves of plants causing them to turn gray and die.
They are most active in hot, dry weather. Several generations are usually
present so traditional controls must be repeated to get all the critters.
(Note: new products can control all life stages in one application.)
There are actually a few insects which cause the familiar "hole in the cane" syndrome. This generally does not hurt the plant, and I don't worry about it too mich. Borers are actually beneficial (other than the holes they make). According to rosarian and entomologist Baldo Villegas, "Cane borers should be considered a beneficial insects equal in rank to the lady beetles and the green lacewings." If the holes bother you, apply Elmer’s glue to cane ends immediately after cutting, which will keep them out.
FungicidesNote: Most fungicides only prevent infestations and cannot cure leaves already infested.
Systemic (single-site) Fungicides
Contact (multi-site) Fungicides
Alternative Spray Products
This closing section describes several eco-friendly solutions and some non-traditional supplemental products. I have not tried many of these so caveat emptor. I have had some success with neem-based products for leaf eaters and insecticidal soap for aphids.
One good source: Gardens Alive – (812)-537-8650
Pyola Insecticidal Spray – uses botanical pyrethrins and rapeseed oil to kill eggs, larva, and adults. Labeled for aphids, leafhoppers, leaf miners, scale, spider mites, thrips, sawfly larva and whiteflies. Also controls caterpillars as well as cucumber and Japanese beetles. Pyola is safe to use on fruit and vegetables right up to harvest day.
Soap-Shield – a new a new all-natural fungicide, combines copper with a naturally occurring fatty acid. On roses, controls black spot, downy and powdery mildew, gray mold and rust.
Also: - Shield-All II (neem as fungicide), Neem-Away (neem as insecticide), many other products.
Other good "natural" products: BioNeem, many Safer and Concern products, and Bonide (a baking-soda-type fungicide). Note: Green Light neem products (e.g., Rose Defense) have the natural insecticide (azarachtin) removed.
These products are not fertilizers, but are supplements to your fertilizing program.
B’Cuzz Root Stimulator is a 100% organic product from Hydrodynamics in Holland. It is a botanical solution that encourages explosive root production. Rosemania highly recommends its usage in all newly planted and transplanted roses. Functions as both a root stimulator and disease inhibitor! (Rosemania)
BioBoost is a new, all-natural biostimulant for plants and soil organisms -- like a megadose of vitamins for your plants. It contains various biological activators (the by-products of a unique fermentation process) such as vitamins, enzymes and other powerful but gentle growth stimulants. (GardensAlive)
SuperThrive "#1 Activator, #1 Reviver, #1 Transplanter, #1 Extra Grower." A classic vitamin-hormone supplement, Superthrive has been used for decades. It’s consistent results as a plant stimulant and tonic are famous. It contains "50 instant usable bio-complexes" which a plant normally has to manufacture. The bottle looks and reads like a bottle of "Texas snake oil", but it really does seem to work.