Roses are among the most favored flowers by gardeners and among the most popular cut flowers. They grow fast and can reward you with colorful, sweet-scented blooms throughout their blooming season. Growing roses has many challenges, including dealing with diseases that affect many varieties.
Rose Mosaic Virus
Prunus Necrotic Ringspot Virus (PNRSV) and Apple Mosaic Virus (ApMV) are two viruses that cause rose mosaic virus. These viruses may appear separately or together on rose bushes. Symptoms usually appear in spring and remain through the growing season and include ringspots, wavy lines, yellow vein banding, oak-leaf pattern and splotches of yellow and green on leaves. The splotches can give the leaves a mosaic appearance, hence the name. Infected buds, scion and rootstocks can transmit the rose mosaic disease through vegetative propagation. The infection will cause the plant to grow less vigorously, produce few flowers and possibly die during the winter.
Crown gall, caused by Agrobacterium tumefaciens bacterium, is a disease of rose bushes characterized by rounded galls with a rough, irregular surface. The infection appears initially as small protuberances (bulge or tube that protrudes) on the plant's surface. Young galls are light green or white with soft tissue and turn dark and woody as they become older.
The bacterium enters the plants through wounds caused by pruning, grafting, chewing insects or from any mechanical injuries. Crown gall can cause stunted growth, weakening, poor foliage and few blossoms on infected roses. When a single gall appears at the base, it can have more impact to the plant than several galls on the roots or canes.
Peronospora sparsa is a fungus that causes downey mildew. Characterized by purplish, red or dark brown spots on leaves, downy mildew happens in moist, humid conditions. Downey fungal growth appears at the leaf undersides, causing yellowing and dropping of leaves. The fungus produces spores only on living plants. It can also produce resistant spores that can survive even in unfavorable conditions. Reducing humidity and improving air circulation around plants can help prevent downey mildew.
Caused by the fungus Diplocarpon rosae, black spot is the most important disease of roses worldwide, according to the University of Nebraska at Lincoln. It is a very serious disease which can severely weaken plants and lead to winter injury or death. Black, circular spots with feathery margins, measuring up to one-half inch, appear on the upper leaf surfaces. Infected leaves may turn yellow and drop from the plant. The infection appears on the lower leaves first, then on middle and upper leaves. Black spots cause excessive defoliation, which reduces stem length, size, number and quality of leaves and blossoms. Symptoms can also manifest on canes, appearing as raised purple blotches that later become blackened and blistered. Wet weather is favorable for the disease to develop and spread. Splashing water and wind spread spores.
Botrytis blight, or gray mold, caused by Botrytis cinerea, can affect just about all parts of a plant with the exception of the roots. It appears initially as brown spots on rose buds and petals that develop into masses of silver-gray spores on dead tissues of plants. Infected flowers turn into a brown, soggy mush and may fail to open. Twigs may die back. Wet, humid conditions favor this disease.