Diseases of flowers
Black Spot
Black spot is a fungus that causes circular black patches surrounded by yellow halos, up to 1/2” across, on rose leaves. Leaves eventually turn yellow and die. Some roses, such as Persian Yellow and Austrian Copper, are more susceptible; most of the Explorer series are resistant.
- Cultural control - fungus over winters in dead leaves on the soil, so removing debris in the fall is important. Water in the morning, and water soil, not leaves if possible.
- Chemical control - systemic fungicides such as Funginex (liquid) and Benomyl (wettable powder), and the contact fungicide Folpet (wettable powder) will control it if used as soon as disease is seen. It is important to alternate fungicides, as fungus builds up a tolerance to specific chemicals.
Botrytis Fungus
Botrytis fungus causes young shoots of peonies to wilt and fall over, and a black rot develops at the base of the plant. Gray mold forms on stems just above soil level, and flower buds wither, turn dark and die. Open flowers can also be attacked - they turn brown and the leaves become covered in gray mold, with irregular brown patches.
- Cultural control - remove all leaves from plant to just below ground level in the fall, increase air circulation and available sunlight by pruning and/or thinning plants.
- Chemical control - apply a systemic fungicide such as Benomyl as soon as shoot growth appears in the spring to plants that were affected the year before.
Clematis Wilt
Clematis wilt is a fungus which causes wilting and die-back of shoots in the large-flowering varieties of clematis. It affects plants just above ground level, and young leaves are affected first - eventually the whole stem turns brown and wilts. Stems can’t recover, but it rarely affects the roots.
- Cultural control - none
- Chemical control - an infected stem can be cut back to soil level, or just below, treated with a systemic fungicide such as Benomyl two or three times at 10 day intervals as soon as the problem is noticed, and a new stem will grow.
Bacterial Leaf Spot
Bacterial leaf spot and stem rot affects geraniums, causing two entirely different symptoms. Small, water-soaked spots develop on the underside of leaves, the spots spread, and turn dark brown and dry. Bacteria enters the stem, causing stem rot where leaves join the stem, and leaves wilt and fall off. Another symptom is wilting of leaf margins, which eventually spreads to form large, dead areas enclosed by leaf veins. Stems become black, dry and shriveled, and yellow bacterial ooze is visible when they are cut. Infected cuttings fail to root, but develop a rot which moves up the stem. Bacterial stem rot moves more slowly than damping off, which can kill a plant very quickly.
- Cultural control - spread on tools, soil, physical contact, insects, so sanitation is important. Plant disease-free stock, root in individual pots so that infected plants can be easily eliminated.
- Chemical control