Apple scab, a fungal disease, does more damage to apples internationally than any other plant disease. Apple scab and other fungal diseases that infect apples can be treated with many commercially available fungicides with the result of a bigger harvest of larger, healthier fruit. But the grower needs to understand how these diseases develop and spread, and which fungicide works for a particular infection.
Main Fungal Diseases
Learning the botanical names of fungal diseases is important because popular names are confusing. If your apple tree exhibits symptoms of a certain fungal disease and you research the proper treatment, you want to make sure you are treating the correct fungus.
The main fungal diseases that attack apples are apple scab (Venturia inaequalis), powdery mildew (Podosphaera leucotricha), cedar apple rust (Gymnosporangium), sooty blotch (Glosodes pomigena), black rot (Botryosphaeria obtusa), bitter rot (Glomerella cingulata), and fly speck (Microthyriella rubi) and blister spot (Gloeodes pomigena).
Effects on Appearance and Growth
When fungal infections are not treated, the damage to the appearance and size of apples is wide-ranging. Apple scab and cedar apple rust cause unattractive, raised lesions, bumps and scars on the apple. Rots can cause ugly, brown decaying spots on the fruit. Other fungi can cause small or deformed apples. Fungicides applied following directions on the label can yield larger, better-looking apples.
Scholars at the University of Arkansas reviewed the literature of numerous clinical studies on the effects of fungicides on health. Botanical and medical scholars concluded that fungicides that come into contact with the skin or are inhaled can cause skin allergies and mucous membrane irritations. However, unchecked fungal growth in apples can cause the growth of toxic mycotoxins. Many countries in the developing world have suffered epidemic health problems with mycotoxins. On balance, scholars recommended a cautious and judicious use of fungicides.
The most useful way to distinguish the numerous commercial fungicides on the market is to look on the labels to see exactly what they contain that kills fungi and which fungi they kill. Some chemicals are more effective for specific types of fungi than others. Both the United States and Canada require that the active ingredients be listed.
The Bordeaux mixture, a combination of copper sulphate and hydrated lime, is effective against both apple scab and downy mildew, and is highly recommended by horticulturalists at the University of California, Davis, among others. It not a commercial brand; anybody can make it. The name comes from its origins in the French vineyards of the Bordeaux region more than a century ago.
The Ontario Ministry of Agriculture Food & Rural Affairs in Canada recommends these fungicides as being safe and useful against specific fungal infections:
Captan, dithane + karathane, kresoxim-methyl, mancozeb, and manzate will treat apple scab. Dodine and flusilazole are also useful, but have been resisted in some orchards.
Captan, Kresoxim-methyl, trifloxystrobin work against Black rot, Bitter rot, Sooty blotch, and Fly speck. Dithane + karathane will suppress powdery mildew, sooty blotch and fly speck. flusilazole works best against cedar apple rust. Myclobutanil is recommended for cedar apple rust and powdery mildew.
Healthy apples with less fungicide
Fungicides can help give you blemish-free apples, yes, but it is always better to reduce the need to use chemicals. The first step to preventing fungal disease to ensure healthy apples is to understand how fungi infect your trees and how they spread.
Fungi grow in humid conditions and in moisture. Spores that infect blossoms and branches ordinarily spread by wind and rain splash in the spring. Root rots spread by water in the soil and work their way up the trunk of the tree into the branches.
Do not plant apple trees next to trees infected by fungal disease or close to older orchards.Thin the crowded interiors of your trees to let in the sun and wind and help dry the apples after it rains. Water at the roots to prevent wet leaves that are susceptible to fungi. Keep your soil well drained to prevent root rot.