Apple scab is the number one threat to a grower’s apple crop. If the ascomycete fungus (Venturia inaequalis) isn’t controlled and the growing conditions are ripe for its development, crop losses in excess of 70 percent are to be expected.
The fungus largely overwinters on the orchard floor amidst fallen leaves. Humid, wet, cool weather during the spring months supports and promotes its growth and the production of pseudothecia. These microscopic structures contain sacs of ascophores, the primary inoculum in the spring. When it rains and temperatures are above 40 degrees, the ascophores eject and air currents carry them to emerging leaves. A finite number of ascophores are released over the course of several weeks, beyond petal fall. When this period has passed, there is no longer a risk of primary infection. Once the fungus gets established on a tree’s leaves, however, conidia form on the lesion surface and become a secondary form of transmission for the remaining growing season. It is these secondary infections that cause extensive damage to the apple crop.
Controlling this fungus is a challenge to organic and conventional growers alike. For those just starting out or interested in expanding their orchard, there are at least two dozen cultivars that demonstrate strong resistance to apple scab. They range from Britegold to Williams Pride and include the increasingly popular Liberty, which also demonstrates resistance to cedar apple rust and fire blight. The marketability of these scab-resistant cultivars can be a little iffy, as consumers and processors may not be willing to shift from their favored (but scab-susceptible) Cortland, Empire and McIntosh. This is where the organic apple grower must step up to the plate and effectively communicate the value of these resistant varieties. Organic apple eaters are dedicated lots, and it will likely be shown over time that they are more committed to an organically produced apple than a known variety that is conventionally produced.
If you have an established orchard and it is not comprised of apple scab-resistant cultivars, the value of thorough and diligent sanitation practices cannot be overemphasized. There is a finite number of ascophores. If they are eliminated or prevented from ejecting, the risk of infection for the remaining growing season becomes nearly insignificant. Because the fungus overwinters in diseased matter on the orchard floor, removing and destroying fallen leaves and drops at the end of the season will go a long way toward reducing future infection rates. Without a doubt, this is a tall order for mid and large-scale growers. There are a number of leaf blowers and vacuums on the market, and blowing the leaves into windrows and mechanically shredding them to encourage their decomposition is an option. Another option is flail mowing, which has been shown to reduce the fungus’ presence when performed in the fall. Along these lines, many studies have looked for compounds that would further stimulate leaf decomposition. Research by the Applied Plant Research and Plant Research International institutes based in the Netherlands’ Wageningen University has shown autumn application of vinasse, the fermented waste product of the sugar processing industry, reduces ascospore formation by more than 95 percent. It also speeds decomposition over the winter. Healthy soil has a strong, hearty biological community that will also speed leaf decomposition. Make sure you take adequate steps to add organic matter to your orchard floor, in the form of livestock manure or compost, to encourage that community.
Even if extensive preventative measures are taken, a small portion of the fungus will likely get past you, develop and infect your trees. For this reason, you may deem it necessary to use a fungicide to protect against secondary infections. Lime sulfur, wettable sulfur, copper sulfate and Bordeaux mix (copper sulfate combined with hydrated lime) are synthetic substances federally approved for use in organic agriculture. These fungicides are best used as protectants and applied when the conditions are ripe for infection. They cannot halt an infection once it has taken hold.
Copper-based fungicides are effective, but also potential toxins. The NOP stipulates copper sulfate must be used in a manner that minimizes its buildup in the soil. Copper does not degrade in the soil, and there is international concern over the long-term effects of its use in agriculture. High levels of copper are toxic to soil and aquatic organisms, plants and humans. The Netherlands and Denmark have banned its use in agriculture, and the European Union is in the process of phasing it out. The EU project Replacement of Copper in Organic Production, or REPCO, was created to develop copper fungicide replacements to control downy mildew (Plasmopara viticola) on grapevines and scab in apple trees. Over 100 plant extracts and oils, materials derived from microorganisms and other materials were analyzed and tested. About 70 showed promise in the lab. In the orchard, yucca extract produced significant results (comparable to copper) when used alone, and that effectiveness increased when it was combined with sulfur. The plans are to produce it commercially, but it could be five years before the product is made available.
The REPCO study also found potassium bicarbonate, when combined with sulfur, had an efficacy comparable to copper hydroxide: incidence of scab on apples was 39.9 percent for copper hydroxide and 35 percent for potassium bicarbonate. The downside is that trees treated with potassium bicarbonate also exhibited high levels of phototoxicity on flowers, and fruits treated with it were significantly russeted.
There is a lot of exciting, ongoing research when it comes to organic apple production. Through the development of scab-resistant cultivars and discovery of natural fungicides, our children may one day consider apple scab the least of their concerns. Conventionally grown apples consistently rank high in studies, when it comes to the number and amount of pesticide residues on them. More and more consumers are reading these study results, and we may see a rise over time with respect to organic apple demand and tolerance of moderately scabbed apples.
The author, a monthly contributor to Growing, is a biologist who lives and farms in Vermont’s Northeast Kingdom.