Fall 1995 (v7n4)
Paul Vossen and Doug Gubler
Adapted from Plant Protection Quarterly 5(2):17. 1995.
Editor's note: This article is adapted from the UC Plant Protection Quarterly, Volume 5, Number 2. It describes experiments conducted in 1993 and 1994. Vossen reports that similar results were obtained for the 1995 season. The full summary of data is too large to include here; contact the author for detailed results.
Apple scab, Venturia inaequalis, is a serious fungal disease on apples in the North Coast region of California. It is particularly severe during early spring when leaves, flowers, and fruit are developing. If not adequately controlled, especially in rainy years, scab can cause almost total destruction of an apple crop and defoliation of trees. Growers typically apply conventional fungicides at seven to ten day intervals, starting at the green tip stage, to prevent infection of susceptible tissue. There are many fungicides that provide temporary protection against apple cab and some provide a short term curative action (kickback) on leaves that have already become infected.
Some modern fungicides offer more than 10 days of protection, over 96 hours of kickback activity, and a minimum of phytotoxicity. They can be used after fruit formation without causing fruit russeting, a chemical burn of the fruit skin. Prior to their development, liquid lime sulfur was a commonly used fungicide. It provides approximately five days of protection, 36 to 72 hours of kickback activity, but is risky to use after fruit formation because it can burn foliage and fruit if applied during warm weather. Liquid lime sulfur and other alternative materials are currently staging a resurgence because of their status as products that can be used in an "organic" production system.
Food safety and environmental protection are two very important issues to today's consumer. Therefore, the use of conventional pesticides has come under increased scrutiny and the use of alternatives under increased demand. Many North Coast apple growers are trying to produce apples organically to take advantage of higher market prices brought about by demand for environmentally sensitive fruit production methods. Apple scab has limited the ability of these growers to produce organicallygrown apples economically due to yields losses when scab causes flower drop and lower prices for blemished, misshapen, and smaller fruit.
Materials which are legally classified as "organic" by the state health and safety code and Organic Food Act of 1990 were tested in these experiments and include fixed coppers (Kocide & COCS), wettable micronized sulfur (Thiolux), lime sulfurs (Liquid Lime Sulfur & Orthorix), mineral oils (Hytech & Stylet), insecticidal soap (MPede) and nutritional supplements (Stoma Feast and Compost Tea).
Many growers have used several of these materials to control apple scab, but without direct comparisons. Fixed coppers have never been used much because they can cause fruit russeting. The various sulfur materials have been used for years. Oils and insecticidal soaps have been effective in controlling powdery mildew on some plants and in controlling some insect pests. Stoma Feast is a multimineral supplement in a gelatinous base and when combined with fixed copper and sulfur, it is claimed to enhance their effectiveness. Compost tea is made by bubbling air for 21 days through a mixture of water and finished compost made from dairy manure. Brewer's yeast is added to the liquid 24 hours prior to use. It looks like thinneddown molasses with an agreeable organic odor. Compost tea presumably coats leaves with antagonistic fungi and bacteria and/or provides a nutritional stimulation to leaves, flowers, and fruit and thus prevents apple scab infection.
The conventional fungicides Captan, Ziram, Funginex, Benlate, and Topsin have been used for scab control for many years. These, plus two relatively new materials, Rally and Rubigan, control scab but cannot be used in an organic system. Fluazinam is a nonregistered new product for control of apple scab. Benlate and Topsin have not been used for several years because resistance has developed by the fungus, and thus they were not included in this trial.
The experiments tested the efficacy of currently available organic chemicals in comparison to several conventional fungicides. Four different orchards were selected, each with a history of severe apple scab infection.
Overall Comparisons
After two years of experiments and several years of observations, we have come to a number of conclusions about organic materials for control of apple scab.
Lime sulfur is difficult to work with, especially out of large drums. It cntrols scab moderately well in the early part of the season but not as well as Thiolux or fixed coppers. When used after fruit formation it causes russeting on fruit even at a lower rate (1 gallon per 100 gallons of water). It can be used to eradicate or give kickback control of infections within 36 to 72 hours of the infection period.
Orthorix is very similar to lime sulfur but somewhat easier to work with. It is used at a much lower rate since it is formulated with a surfactant. It has the same tendency to russet fruit.
Thiolux, one of the micronized sulfurs, is easy to work with but requires large quantities per acre (12 to 25 pounds). It performed well in all trials with three to five welltimed applications. Starting at the high rate and finishing at the lower rate when fruit is formed, it did not cause fruit russeting.
Kocide performed well in controlling scab infection on fruit and leaves and is very easy to work with. At higher rates (4 pounds per acre and above), it caused severe fruit russeting. At 0.5 pounds per acre, it controlled scab without severe fruit russeting. It also could be combined with Stylet oil or Thiolux and used at a lower rate without fruit russeting and perhaps enhancing control. Registration is pending in California. COCS performed similar to Kocide in all experiments.
Stylet Oil and Hytech Oil showed some efficacy in controlling apple scab. Stylet oil was more effective in combination with one of the fixed coppers. Both were somewhat difficult to work with since they were heavy liquids used at fairly high rates.
MPede, an insecticidal soap, showed some ability to prevent scab infection, and was similar to the mineral oils.
Stoma Feast tended to congeal in the mixing process and plug the sprayer filter, thereby altering spray nozzle emissions. Its effectiveness in enhancing control is not known at this time.
Compost Tea was not effective in preventing scab infection and in some cases appeared to enhance apple scab.
Timing of spring spray applications is a very important factor in apple scab control since the fungicides used offer only temporary protection or limited kickback activity. New tissue that is generated by the tree during this rapid growth phase must be covered with fungicide to prevent crop loss.
Most of the conventional fungicides used in these trials did an excellent job in preventing apple scab infection. Several of the organic materials have now been documented as potential alternatives by also providing excellent control of scab. These materials can be used for the purpose of lowering the environmental impact of more toxic fungicides, to reduce the threat of resistance when alternated with conventional fungicides, and to conform to organic certification laws.
For more information write to: Paul Vossen, UC Cooperative Extension, 2604 Ventura Ave., Room 100, Santa Rosa, CA 95403.
(CIAPP.019)
Contributed by Paul Vossen
[ Back | Search | Feedback ]