Orange-rust blisters on the undersides of leaves, a velvety white powder over stems, black spots spreading across yellowing foliage: rose trees are among the most beautiful flowering shrubs, and among the most vulnerable to disease. Give them space and air circulation among the leaves, and regular organic care will keep most of these troubles at bay.
Although they may be fragile, their diseases can be mastered!
Regular and often organic care helps eradicate or prevent many rose tree parasites and diseases.
Aphids on rose trees
Aphids disturb rose tree growth and its blooming
Aphids are one of the major parasites, coming to disturb rose tree growth and often its blooming, too.
Here a few solutions to get rid of aphids naturally on rose trees:
Ladybugs
Since ladybugs eat aphid larvae, they naturally contribute to controlling aphid populations. This method is increasingly practiced by professionals who manage parks, and it is 100% organic.
Made with Marseille soap or beldi soap dissolved in water and sprayed on the plant, this soap mix makes it hard for aphids to stick to leaves.
Grate 5 oz (150 grams) of soap and dissolve it in 1 quart (1 liter) water, add and mix in 1 tablespoon vegetable oil. Spray on plants.
Check the undersides of the lower, older leaves first, since rose rust pustules often appear there earliest, but also inspect canes and stems, where early-season infections can show up.
Rake up and bin every fallen rose leaf beneath the bush, since spores survive on the litter and splash back onto new growth with the next rain.
Go easy on high-nitrogen feeds, as the soft sappy growth they force is far more readily infected than firm, potassium-fed shoots.
Rose tree chlorosis
This is one of the major issues that rose trees face, which has distinctive symptoms: yellowing of leaves that follows leaf discoloration.
Rose tree chlorosis is connected to the soil, often too poor, heavy and chalky. This prohibits the shrub from extracting much-needed iron from the soil.
Symptoms are very clear, and treatment is quite easy to apply.
Every winter, mix horse manure into the soil. If the disease is already apparent, there are anti-chlorosis treatments for sale in garden stores that help the rose tree to harvest the iron it needs.
On chalky, alkaline ground, choose an iron chelate labelled Fe-EDDHA, which stays stable at high pH, whereas Fe-EDTA and Fe-DTPA release much of their iron in such soils and work less reliably.
Powdery mildew and rose trees
Also called white powder disease, powdery mildew is a fungus that develops a layer of white velvety powder that covers leaves and stems.
It develops particularly fast when weather conditions are hot and humid.
Months that are most favorable to it are April or May, when temperatures rise and the air is still full of moisture from the wet months of March and April.
It also makes a comeback at the beginning of fall, when nights grow longer and moisture once again increases.
Ensure proper air circulation among your rose trees to avoid stagnating moisture.
Powdery mildew spreads when rose trees are planted too close together, and/or when leaves have trouble finding air…
Remove infected portions immediately.
Burn diseased leaves and plant parts.
Avoid overwatering.
Eventually, spray with fungicides.
Milk. Mix ½ quart (½ liter) milk to 4½ quarts (4½ liter) water (total 5 quarts (5 liters)), spray every week until the disease disappears!
