Powdery Mildew - Powdery Mildew caused by the fungus Sphaerotheca pannosa var. Rosa, is a white powdery fungus that appears on the leaves and new growth of the rose causing the new leaves to curl and die. It affects roses that have dry roots particularly roses planted under eaves or planted next to a fence and is caused by humidity and is worse when the weather changes to colder nights and warm humid days. Poor air flow between plants causes the mildew to form also. Spray with full cream milk mixed one part to six parts water. The fat suffocates the spores. Ensure good air ventilation or spray with Mancozeb.
Black Spot - Black Spot is a fungal disease caused by the fungus Diplocarpon rosae which turns the leaves yellow with black spots. These leaves once infected fall off the bush and the plants capacity to produce good blooms is seriously reduced. The disease is caused by the humidity in the air and poor air circulation and is spread by spores that live in the soil or carried in the moist air or by water that has been in contact with affected leaves. Spores from affected leaves and ground matter can contaminate gardening equipment causing the disease to spread further. As soon as the Black Spot is noticed, the affected leaves should be removed from both the bush and from the ground. To prevent the disease from spreading further throughout the soil a thick layer of mulch followed by a layer of wood chips will prevent the spores from spreading from the soil to the foliage of the rose. Spray for Black Spot twice a year with Lime Sulphur, once at the end of Autumn when leaves have fallen and again in Spring once leaves have opened. Spraying with Trioforine, Mancozeb and Kocide Blue alternating them as needed throughout Spring and Summer will help keep manage the disease. Also avoid watering leaves late in the day.
Spider Mites, Aphids and Thrips - The prevention of these insects from attacking the rose bushes can be done by spraying with Pyrethrum or confidor. Follow the directions given for usage and wear appropriate safety gear when spraying.
Rose Scale - Rose Scale is a small insect that has a hard outer shell brown or white. The scale often appear on older branches and canes sucking the sap from the rose stems and spreads along the cane and underneath the foliage leaving a residue. The plant becomes weak if left untreated and scale will eventually kill the plant. Usually scale occurs in areas that are warm, dry and protected under the eaves of the house or against a fence. White Oil is recommended to control the scale.
Rust - Rust is caused by the fungus Phragmidium disciflorum encouraged by warm humid weather. Small red dusty spots form on the branches and leaves eventually turning black causing the leaves to dry out and fall off the bush. If the leaves are not removed the spores will over winter in the soil ready to attack in the new season.By spraying with a fungicide like Mancozeb it will give some control of the fungus Rust usually affects weak growing bushes.
Nutrient Deficiencies - Nutrient deficiencies are evident in the rose garden by the leaves becoming yellowed with a distinct veined pattern on the leaves. The leaves become bright yellow and then fall of the bush. Flower and new growth production is very poor and the rose will remain in a dormant state until such time some nutrients are given. Compacted soil will also contribute to poor growing roses as aerated soil allows water to penetrate to the roots and assist with sap flow throughout the plant delivering the essential nutrients required. If you have roses that are not performing check the ph level of the soil it should be between 6.5 and 7 and ensure that the soil is lightly dug up around the bush not disturbing the roots and then feed a balanced food for roses. If the Ph is not right provide lime or gypsum to make the soil balanced. Ph levels higher than 7 are too acidic and ph levels lower than 6.5 indicates alkaline.