Before even looking at applications to treat plant diseases, try and prevent initial infection by being wise about proper garden hygiene. Constant and complete soil cultivation will vastly reduce the risks of disease occurrence. Choosing vigorous and hardy plants will also safeguard against a range of diseases. Many plant producers subscribe to virus-free certification schemes. So, when you come across them, it is a great idea to use only certified specimens.
The majority of chemicals that deal with disease are preventative only. So when disease is identified make absolutely sure that you cut out affected areas of the plant or grub out entire plants if the disease has got out of control. Never fail to thoroughly burn all resultant debris.
Keep plants in the prime of health and nourish them well. This will make them more resistant to disease attacks. But, if disease does occur, here is an alphabetical guide to identifying diseases and the correct methods of treatment.
Disease: blackspot
Attacks: roses
Recognizable symptoms: dark, blotchy spots on leaves which will then appear anemic and
ultimately fall off
Treatment: spray often with captan, zineb or benomyl
Disease: blight
Attacks: potatoes and tomatoes
Recognizable symptoms: leaves become blotchy, turning brown and rotting; grey patches
observable on tubers
Treatment: spray every 14 days with liquid copper, zineb, maneb or a Bordeaux mixture
Disease: botrytis (gray mold)
Attacks: multifarious plants but has a penchant for strawberries and tomatoes
Recognizable symptoms: leaves rot, covered in a velvety mold
Treatment: get onto this problem as soon as you spot it; a general purpose garden fungicide is
ideal
Disease: cane spot
Attacks: blackberries, loganberries and raspberries
Recognizable symptoms: regular round dark spots appear on the stems and will then turn white and crack open
Treatment: dilute water with 5% lime-sulphur and spray as buds first appear, reduce dilution to 2.5% and spray as blossoms appear
Disease: chocolate spot
Attacks: broad beans
Recognizable symptoms: blotchy brown marks on leaves during the summer and likewise after early winter frosts
Treatment: spray immature leaves with a copper-based fungicide
Disease: club root
Attacks: predominantly brassicas, but also wallflowers and stocks
Recognizable symptoms: wilting foliage and distorted, swollen roots
Treatment: apply lime liberally to surrounding soil; dust holes with 4% calomel before planting
Disease: damping off
Attacks: a variety of herbaceous plants but singles out lettuces
Recognizable symptoms: seedlings wilt, rot and ultimately collapse
Treatment: diluted Cheshunt compound, zineb or captan
Disease: peach leaf curl
Attacks: almonds, nectarines and peaches, even ornamental non-fruiting strains
Recognizable symptoms: red blisters on foliage, turning white with time and then brown; leaves ultimately fall off
Treatment: in mid-winter, spray with Bordeaux mixture or lime-sulphur
Disease: petal blight
Attacks: dahlias and anemones, but saves its worst for chrysanthemums
Recognizable symptoms: petals brown off in wet periods
Treatment: use zineb immediately before flower buds open
Disease: powdery mildew
Attacks: an absolute multitude of plant species
Recognizable symptoms: a pale, powdery coating on leaves and shoots
Treatment: spray frequently with dinocap or benomyl
Disease: rust
Attacks: numerous flowers and vegetables
Recognizable symptoms: orange colored spores on foliage and stems
Treatment: mist often with zineb (once a fortnight or once a month, depending on seriousness of disease)
Disease: scab
Attacks: apples and pears
Recognizable symptoms: brown scabs on fruit
Treatment: captan, benomyl and lime-sulphur are all effective
Disease: tulip fire
Attacks: tulips
Recognizable symptoms: shoots rot and the bulbs become covered in dense black spores
Treatment: just before planting out, sprinkle bulbs and soil with quintozene
Bear in mind that most diseases cannot be eradicated without severe intervention by cutting out affected areas, but equally that diseases may be controlled by correct but not over aggressive treatment with organic products. There is a solution for virtually every problem, so the most important attribute to have is an eagle eye. Spot and identify problems early and you won´t go far wrong.