How to: Treat Diseases on Roses


Managing Common Diseases and Pests:

Spring is far enough along now that I noticed my first problems troubling my roses the other day - aphids, and I knew it was time to get this post out so you can take care of your roses, too.

Roses are some of the most susceptible plants in your garden to disease and pests. If it's not blackspot, it's powdery mildew or aphids mowing down all your new growth.

This is never a bonus... unless you consider that roses are a great indicator for you to notice and manage disease and pests in your whole garden. They are generally the first to be attacked, so use this to your advantage.

When your rose gets hit by disease, treat the bush immediately.

Powdery Mildew:

Powdery mildew looks just like its name: grey-white powder on leaves, stems, sometimes petals. It can eventaully cause the leaves to disfigure and fall off, and in severe enough cases, it can kill an entire plant. It is caused by a combination of cool, humid evenings and hot, dry days, plus the presence of mildew spores. If one of your roses is infected, the rest can become so. Roses with glossy leaves have a little more resistance to it, but can still become infected.

When you first see signs of powdery mildew, you can attack it organically in a few different ways.

1. At midday on a sunny day, hose mildew off of leaves entirely, taking care wash all signs of it from the entire plant. The sun will dry the plant out by the end of the day. This is my least favorite option for a few reasons: you risk spreading mildew to uninfected areas of the plant with the spray, and you risk sunburn of the plant by watering leaves in the middle of the day.

2. Treat the infected areas carefully and sparingly by spraing with a solution of 1tsp of baking soda or sodium bicarbonate to 1 quart of water. Baking soda and sodium bicarbonate can have negative effects on your soil, so be cautious with this solution.

3. Treat the infected areas with plant-based oils like neem oil or jojoba oil. Or use a combination of neem and baking soda, diluted in water. Again, use this spray sparingly.

Rust:

Rust is caused by cool, wet weather, and will leave tiny rust-colored speckles along your leaves. Leaves may eventually discolor and drop.

The best organic way to treat rust is to pull off any affected leaflets when you see them and discard them in the trash. You can treat with fungicides if the problem gets bad enough. If rust is an extremely persistent and extensive problem on a rose, consider replacing it with a less troublesome cultivar.

Black Spot:

No, this is not some fabled pirate disease. This is a terribly common disease among roses, caused by a fungus. It appears as small black spots on the leaflets of a rose, eventually growing in size and discoloring the leaflets to yellow, before causing the entire set of leaflets to fall off. It can be spread by contact and through the soil, so take care when treating this disease. This fungus requires water to grow, so by keeping everything of the rose above the soil dry, you can do a lot in the way of prevention.

Treat it organically by:

1. Pulling off any affected leaflets and discarding them in a closed trash receptacle. Make sure there are no infected leaves left on the ground.

2. Prevention: do not water plants from above; instead, water down by the roots.

3. Treat persistent infections with solutions of diluted neem oil or sodium bicarbonate, used sparingly.

Nutrient Defficiencies or Excesses:

Too much or too little of a particular nutrient can also be problematic for roses. A typical sign of such an issue would be discoloration of leaves, and eventual leaf drop. To prevent this, fertilize regularly during the growing season with a balanced organic fertilizer for roses, or regularly top dress with compost (add compost like a layer of mulch around the base of the rose).

Aphids:

Along with black spot, aphids are one of the most ubiquitous troublers of roses. Aphids are small, green or yellow, soft-bodied insects that devour new growth on rose bushes. If an infestation is bad enough, it can prevent your rose from blooming all season, as the little varmits will eat off every new bud. Most aphids can reproduce asexually, as many as 12 times per day!

Monitor your roses carefully for aphids, checking the under sides of leaves, too. If an infestation occurs, here are a few organic ways to handle it.

1. Manually remove the aphids, whether by hand and squashing them, or by hosing them off, taking care to get them entirely off of the plant.

2. Introduce (or wait for your latent population of) predatory insects to devour the little rose-devourers. This is the least troublesome option for you, and the healthiest for your garden. Insects that particularly enjoy dining on the soft-bodied aphids are ladybug larvae and adults, lacewing larvae, parasitic wasps, and syrphid fly larvae. Never treat your garden with pesticides; in addition to killing the bad bugs, you'll kill off all of the good ones. Then, when a bad bug inevitably re-enters your garden, its population will surge unchecked because you killed the good guys, who take longer to move back in.

3. Aphids don't like too much light or heat, so mulching with a bright, reflective material (like straw) can deter them. As the summer progresses and temperatures increase, your aphid populations should decline naturally. You can also temporarily try placing reflective tin foil beneath infected bushes, which will discourage aphids from seeking shelter on the preferred underside of rose leaves.

Bristly Roseslug (Sawfly)

These are tiny, pale green wormy creatures in the larval stage that skeletonize the underside of rose leaves. It can do this to an entire bush if it goes unchecked, as it produces several generations per year. It lays its larvae in the soil prior to over-wintering, so it is important to treat the infestations as soon as they occur in spring, so that they don't make it to that stage again in the next season.

You can mitigate damage done by roseslugs by doing a few things.

1. If you catch an infestation early, pick off and kill the little green catepillary-looking insects. Bristly roseslugs have several generations per year, so if you kill the first generation, you should solve your problem.

2. At the end of the growing season, once the rose goes dormant, pull off any remaning leaves (and pick up any from the ground) and put them in a closed trash recepticle. Roseslugs lay their eggs on the ribs of the rose leaflets, so by removing these, you remove the eggs. The odd thing is that the adults are beneficial pollinators. Their young are the problematic ones.

Prevention is Your Best Medicine

The best way to keep your roses is definitely by taking preventatie measures.

1. Plant roses in full sun locations with well-draining soil, in a well-ventilated area.

2. Pick up any fallen leaves or other debris from roses and discard in a closed trash receptacle.

3. Pull off any infected leaves and dispose of them in the same way - in a closed trash receptacle.

4. Water from the ground, down by the roots. Avoid spraing the leaves of roses.

5. Monitor your roses regularly for the first signs of trouble.

References:

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7463.html

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7404.html

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