MSU Organic Apple Project: Pest and Disease Management
Apple growers east of the Rocky Mountains are faced with several arthropod and disease complexes, which are the “Achilles heel” of production and make low-input production especially difficult. The most devastating pests in organic apple orchards in Michigan are plum curculio, Conotrachelus nenuphar , codling most, Cydia pomonella , fire blight, Erwinia Amylovara and apple scab, Venturia inaequalis . Without adequate control these pests can render a crop worthless within one growing season and occur within the fruit at harvest. In addition to these major pests there are also many minor pests that can impact fruit set, tree health and harvest. The emphasis in organic production has been on biological control through beneficial organisms (predators and parasitoids) that stabilize prey/pest densities below the economic threshold and on soft-sprays that do not harm beneficial organisms.
The Clarksville Horticulture Experiment Station's Organic Apple block had little to no fruit set in the 2002 season due to low spring temperatures and poor pollination conditions. We experimented with four strategies for disease control that helped us to develop a disease strategy. Sprays of sulfur or lime sulfur provided reductions in apple scab infections but it was clear that more background work was necessary before we could consider compost tea. Insect scouting and trapping with minimal sprays provided a base year of data describing insect populations. Based on the 2002 season, a plan was proposed and adopted for the 2003 season. The North block and Rootstock block are receiving sprays based on economic threshold and monitoring for beneficial/pest ratios. These blocks will receive a minimum of sprays that ensure a healthy end product while minimizing harm to beneficials. The South block will rely more heavily on biological control and will test the ability of predators and parasitoids to control pests.
Worm Management
Lepidoptera is the order of insects knows as “the butterflies and moths”. This includes the codling moth, oriental fruit moth and leafroller apple pests. The codling moth can be especially damaging to apple trees. Their larvae cause damage to apples throughout the season by burrowing into them and excreting frass. Oriental fruit moth damage is very similar to that of the codling moth, but typically much less severe. Leafrollers cause damage by rolling leaves to hide from predators and rough environmental conditions and then eating the leaf from the inside out. This rolling habit makes them difficult to control with sprays. A fairly new method of controlling lepidoptera without spraying is pheromone mating disruption. Pheromone mating disruption involves saturating an area with female sex pheromone so that males do not successfully find and mate with the female moths. Reduced mating results in fewer offspring and the population are maintained below economic thresholds. Mating disruption works best on large areas usually 5 acres or greater with few empty spaces. The technology is most effective in orchards with moderate to low insect pressure. It is important to maintain a trapping program to ensure the mating disruption is working properly. Pheromone traps should be hung high on the outer perimeter of the tree canopy so that moths can gain access to the trap opening. They should be buried in the foliage! Check and clean traps on a weekly basis.
For the 2003 growing season, all the Organic Apple Project blocks were treated with Isomate CM/OFM/LR for mating disruption of codling moth, oriental fruit moth, and leafrolllers. A novel hollow twin-tube design dispenser is employed for the application and release of the active pheromone. This new multi-species is experimental and not commercially available to date. The dispensers were applied at a rate of 400 per acre just prior to bloom targeting the first flight of codling moth. The pheromone is released at a constant rate for the duration of the growing season. The twin-tube design allows easy application by spreading the tubes apart and slipping the dispenser over a limb within 3 feet of the top of the tree canopy. The cost of this product is approximately $150.00 per acre and must be applied at a full rate.
In addition to mating disruption, Bacillus thuringiensis (Bt) will be sprayed as needed for leafroller control and granulosis virus will be sprayed as needed for codling moth control. These are both biological control agents that must be specifically timed for effective control. Granulosis virus is a natural disease of codling moth larvae and must be timed to coincide with hatching larvae before they enter fruit. Leafroller sprays should be put on before they roll leaves.
Plum Curculio Weevil Management
Coleoptera is the order of insects that we call the “beetles”. Plum curculio (PC) is the most damaging beetle in apples. While they have only one generation per year in Michigan, they are long-lived and a single individual has a high capacity for yield-reducing damage. Both males and females consume fruit, but most of the crop damage is from female oviposition is crescent-shaped and, in apple, has a brown, fanlike appearance at harvest. If the larvae develop in the fruit, there is a tunneling damage inside the fruit- often causing the fruit to drop off from the tree. If fruit remain on the tree, the damage can significantly reduce the market value of the crop. Currently, there are no sanctioned organic control strategies for plum curculio. The historical means of conventional control has been calendar sprays of organophosphate insecticides- primarily Guthion (azinphos-methyl.)
Plum curculio activity is difficult to monitor and this is not well understood. A new method for attempting to monitor their activity is modified black pyramid trap. This device is essentially a tree mimic that provides a high contrast, visually attractive target for PC. The design takes advantage of their natural climbing behavior and captures the beetles in a standard cotton boll weevil trap that is placed on the top of the 4-foot tall structure. In addition to visual stimuli, the pyramid traps can be baited with fruit essences (kairomone) that mimic the attractive chemistries of the orchard they are designed to protect. The attractant strategy is effective against both males and females, since both sexes respond to the visual and odor stimuli. Traps are placed throughout the orchard in an attempt to map PC behavior patterns. Trapping may reduce the plum curculio population in an orchard depending on the number of traps. However, additional control measures are usually necessary to maintain PC populations below the economic threshold.
Orchard floor management after June drops is an important element in long-term organic control of PC. The dropped fruit is often infested with larvae. These larvae eventually leave the fruit to pupate, and emerge as adults in August. These second-generation adults over winter, and in the following spring they mate and lay eggs in the fruit. Prompt removal of June drop fruit can help substantially reduce established in-orchard populations the following season.
In addition to trapping and June drop removal, we are utilizing degree-day modeling to time pyrethrum and kaolin clay sprays. Insect growth is dependent on temperature and typically has a minimum environmental temperature requirement for growth to occur. Degree-day modeling helps us to determine the stage and activity of insects based on their temperature units or “degree days” accumulated. Kaolin clay is a substance that is Generally Recognized As Safe (GRAS) by the EPA. It is sprayed on as a liquid that evaporates leaving a particle film, which allows light to pass through it. The particle film creates a barrier that prevents insect feeding and egg-laying. It also makes the tree unrecognizable as a host and provides secondary benefits to trees by reducing the canopy temperature, which reduces heat and water stress. Kaolin clay is sprayed starting early in the season to prevent plum curculio and other insects from damaging fruit. Thorough coverage is necessary for it to be effective. In the North and Rootstock blocks, it will be sprayed throughout the growing season. In the South Block it will be sprayed through 900 degrees-days Base 50 F , which marks the end of the plum curculio season. These sprays will be supplemented with pyrethrum as necessary. Pyrethrum is a broad spectrum insecticide that is broken down rapidly by UV radiation.
Apple Maggot Management
Tephiritidae is the order called the “fruit flies” and includes the apple maggot. The apple maggot is a key pest on apples mainly in the Northeast and upper Midwest. Larvae feeding within the fruit cause their main damage. Two new products containing spinosad are now available for control of the apple maggot. These are Entrust ^TM and GF-120 NF Naturalyte^TM works by attracting flies to baited spinosad, while Entrust ^TM is an organic spray containing spinosad. GF-120 NF Naturalyte^TM will be used as needed in the South block while Entrust will be used as needed in the North and Rootstock blocks. Kaolin clay should also provide control of apple maggot.
Monitoring Insect Populations
Sustainable organic agricultural practices have been observed to increase insect diversity. To monitor insect diversity sampling transects are used. Measurements of insect predators, insect parasitoids, and pests are accomplished with the use of yellow sticky card traps and pitfall traps. Each of the sample transects begin outside of the orchard and plant diversity strips and continue to the middle of the orchard. Monitoring is very labor intensive but crucial to continued understanding of insect populations under organic management.
Disease Management
Fire blight is caused by the bacterium Erwinia amylovora . It is potentially serious threat to the viability of young orchards and has been particularly devastating in the recent past (for more information see http://www.canr.msu.edu/vanburen/fb2000.htm ). Management over both of the organic blocks includes aggressive pruning of infected plant tissue and treatment with copper. Copper treatments have been applied according to blight risk models from first tip. Although copper causes russeting of the fruit, copper treatments were continued past ½ inch green to preserve the integrity of the orchard at the risk of damaging the harvestable fruit this year. Streptomycin is an antibiotic that will be used only in an emergency for fire blight control since it is allowed in organic apple production for this specific purpose.
Apple scab is caused by the fungus Venturia inaequalis . It can cause significant damage in apple trees especially in Michigan. Its occurrence is highly dependent on the temperature and moisture conditions of an orchard. Serenade, sulfur and lime sulfur will be used as needed for control of apple scab. Serenade is a Biofungicide produced by AgraQuest. It is a defined bio-control agent ( Bacillus subtilis QST713) and is applied as a preventative for fungal and bacterial plant pathogens. The sulfur and lime sulfur treatments are standard effective organic fungicide treatments that effectively reduce fungal pathogen pressure.
Developing Plant and Insect Diversity Through “Diversity Strip” Plantings
Part of the Organic Apple Project is focused on insect diversity and its potential role in preventing and maintaining lower levels of pest population. We are consciously attempting to design the ecological diversity around the orchard to enhance and protect insects that are beneficial for the orchard. While this is an intractably complex problem to solve on a complete systematic scale, we are attempting to provide an input fraction that has the highest probability of making a positive impact for pest management. This input is based on increasing plant diversity that is predicted to increase beneficial insect diversity.
Selection of Perennial Plant Species
The plants for our diversity strips along the borders of the orchard were selected primarily for their ability to provide nectar, pollen and refuge for beneficial insects by flowering at different intervals throughout the growing season. These insects include pollinators like bees and bumblebees and parasitoids (small wasps that lay eggs in aphids and lepidotera larvae) and predator insects. Plants that provide for these types of insects generally tend to have open exposed flowers or large heads of multiple open flowers.
The second selection criterion for plants was focused more specifically on the environment around the orchard at this site. Since we are using perennial plants that will not need to be replaced annually, we selected plants that are hardy, tolerant of full sun and the sandy well drained soil that is present at this location. In addition many of the selections are plant varieties that are native to Michigan.
Our planting list is not an absolute standard to follow. We are continually looking for other successes that may fit into our current design. When planting, it is important to keep in mind the functional aspect of ecological diversity rather than aesthetics. Insects are less concerned with artistic arrangements and showy varieties than they are with survival on the edge of a hostile environment.
Another consideration for designed diversity is the time required to establish plants that flower well. Starting plants from seeds is relatively inexpensive and native or wild species are available from Internet suppliers and a variety of catalogs. Unfortunately, seedlings of many perennial species require an entire season or winter vernalization before they bloom. Starter seedlings or mature plants can be purchased from a number of suppliers such as WildTypes (Mason, MI). The mature plants will often bloom in the current season but the cost is significantly higher.
Last year we planted a variety of perennial herbaceous forbs (non-grass) of culinary or medical value. In spite of difficult conditions at planting, several of these plant species survived well to this year and we expect that they will contribute to the species that have been added this year to provide refuge and nectar for our beneficial insects.
Planting and Management Plan
There are four drip-irrigation hoses in the strip. The two outside hoses are lined with comfrey and yarrow plants. This is to help place a border of tough competitive plants between the diversity and potential invading grasses like quack grass. Comfrey and yarrow are also expected to provide early season flowers and a substrate for caterpillars that will be a food source and host for predator and parasitoid wasps. The two inside hoses will be lined with the perennial herbaceous forbs with different flowering intervals designed to span that growing season. The plants are separated into two groups based on potential difficulty in managing spreading by roots or seeds. The more difficult plants are in the row farther from the orchard.
We considered several cultivation, weed, growth and water management strategies including mulching with straw, the use of landscape fabric, and various forms of cultivation. Given the existing drip irrigation system, together with our desire to limit the growth of more aggressive species, we decided to plant in rows that allow routine cultivation between the rows with a self-propelled rototiller. Several plants of each type were established in a group within a row.