Integrated Pest Management

Because many rose diseases are caused by fungi, take precautions to keep the leaves dry and air circulation adequate. Some basic guidelines include:

  • Space plants at least 16" apart at their centers.
  • Water early in the day to ensure the leaves are dry in the evenings.
  • Provide good ventilation in the green houses and hoop houses.
  • Remove any infected plant material immediately.
  • Spay plants with a rotation of fungicide.
  • Inspect plants frequently for signs of disease.
Downy Mildew

Telltale signs include dark brown, purple or reddish spots on the leaves, stems or canes. White growth may appear on the underside of leaves and leaves may turn yellow and drop. It often develops when relative humidity is above 85% and temperatures range from 45-80°F. Because it develops so rapidly—it can infect a rose in three hours of favorable conditions—prevention is key.

Immediately after planting, apply a soil drench with Subdue. Then spray weekly using a rotation of fungicides, taking care to spray the undersides of leaves.

The best defense is to provide good ventilation for the plants and to keep the leaves dry.

  • Provide proper spacing of plants—at least 16" between plant centers.
  • Overhead water early in the day so the leaves dry before cool night temperatures set in.
  • If growing in greenhouses, heat the houses for about two hours during the night. If possible, vent to the outside.

During the early spring, follow these recommendations:

  • Prevent water puddles around the plants.
  • Open vents in greenhouses.
  • Maintain horizontal air flow with fans.
  • In hoop buildings, cut ventilation holes at the sides or raise the skirts, and provide ventilation at the top.

If symptoms of downy mildew appear, remove the affected plants immediately.

Powdery Mildew

Powdery mildew looks like white fuzzy powder and accumulates on leaves and stems. In addition to the white or gray growth on leaves, shoots and buds, other signs include leaf cupping, distorted leaves and purplish blotches on the underside of leaves. New leaves are most susceptible.

Powdery mildew usually develops when daytime temperatures warm to 50-80°F, nights remain cool, and relative humidity exceeds 90% at night.

Preventative measures include:

  • Water early in the day to reduce nighttime humidity levels.
  • Space the plants at least 16" apart at centers to allow for adequate air flow.
  • Spray weekly with a fungicide, making sure to rotate fungicides to avoid developing resistance.
  • Use surfactants with all dry power formulations.

Rust appears as orange or rust-colored growth on the bottom of leaves. Older leaves usually show symptoms before younger leaves. The entire leaf may turn yellow and defoliation may occur. Rust usually develops in mild temperatures and moist conditions. It is spread by wind or water-splashed spores.

Prevention measures include:

  • Avoid getting water on leaves as much as possible—do not let leaves stay wet for longer than 5 hours at one time.
  • Use a rotation of fungicides every 7-10 days.
  • When it rains frequently, increase sprays of fungicides.
  • If the area has a known history of rust, maintain a regular spray schedule.
  • Practice good sanitation: remove any affect plant material immediately.

Because it can spread by wind or water-splashed spores, take care to remove any infected plant material immediately. Avoid getting water on leaves as much as possible and decrease spray intervals during rainy periods.

Black Spot

A rose infected with black spot has round black spots with fringed margins on its leaves or stems. Yellow areas may develop around the spots and leaves may die or drop. This fungus requires water to germinate and infect plants, and can be spread by splashing water. It can survive on dead plant material.

Prevention measures include:

  • Avoid getting water on the leaves and take measures to ensure the leaves dry quickly.
  • Use a rotation of effective treatments every 7-10 days.
  • Increase treatment applications during rainy periods.
  • Practice good sanitation: remove any fallen leaves from pots and surrounding areas; prune and remove any affected plant tissue.

While botrytis can affect many plant parts, it usually damages tender tissues such as flower petals, buds, seedlings, and weakened or injured tissues. The infected plant material turns brown or black, and a gray fungal growth may appear.
It usually develops during prolonged, cool spells when temperatures fall below 60°F and the conditions are moist. The spores can spread rapidly by pruning equipment, and they can survive in decaying plant material and infested soil.

Prevention starts with good sanitation, including:

  • Use sterile potting mix.
  • Store potting mix on a concrete pad or on heavy mil plastic.
  • Use clean implements when pruning, and clean pruning shears, potting machines, potting benches and vehicle beds.
  • Prune out dead plant material.
  • Remove dead leaves from pots and surrounding areas.
  • Remove any plants that show signs of botrytis.

Other preventative measures include:

  • Provide good ventilation.
  • Heat hoop house to keep humidity levels under 90%.
  • Stop overhead watering at least 2 hours before sunset to allow foliage to dry before cool nighttime temperatures set in.
  • Spray with a rotation of effective fungicides on a 7-10 day basis when conditions are favorable for disease development.
Pest Infestations

Inspect your plants for signs of infestations, including mites, thrips and aphids, and beetles and worms.

  1. Mites
    Usually most common during hot weather, mites can be difficult to spot. Because they can be spread by wind, birds, and clothing, including gloves, plants should be inspected regulary.

    To prevent infestation, overhead water during the day. Also, inspect plants weekly by brushing leaves over white paper and examining it with a magnifying glass (10x or 20x). If mites are present, hand-spay webbing from plants. Start at the base of the plant, turn the nozzle up and spray the undersides of the leaves; then turn the nozzle facing down and spray down on the plant. Rotate pesticides on a 7-14 day schedule.

  2. Thrips and Aphids
    Because both thrips and aphids can quickly damage plants and lower their quality, prevention is key. When plants are just beginning to unfurl leaves, treat pests with systemic Marathon (if allowed in your state) or Orthene at 2-week intervals. Spray with Mavrik on alternate weeks.

  3. Beetles and Worms
    Inspect plants regularly for beetles and worms. Often, it's easier to spot black, pellet-like worm droppings on the foliage than it is the worms. Inspect folded over leaves and flower petals too. When present, spray with an effective insecticide.
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