Apple Scab Season Approaches

 Apple scab is the most prevalent and most damaging disease to apples we have in the Midwest. We are now officially emerging into the apple scab season as green tip and 1/4" green leaf tissue is seen on the apple trees.  At this point, scab sprays should be applied according to the spray guide protocol.  In the spring, once temperatures rise above the 42 degree or so  mark, apple scab fungal spores can germinate in water on the surface of  apple tree leaves and eventually,  on the fruit itself. The water or moisture that is on the leaves is termed "leaf wetness". The spores will germinate once the leaves are wet for a certain period of time at temperatures above 42 degrees  On the leaves, olive green to brown spots appear on the site of the infection.  If the leaves have not been protected from this "primary" scab infection, the spores will mature and produce more spores during "leaf wetness" periods and move onto the apples where they form a "scab" like lesion, if the fruit is not protected.  We call the lesions on the apples "secondary" scab.  With enough moisture (leaf wetness), the cycle continues throughout the growing season and destroys the crop.  Each leaf wetness event at the proper temperature that occurs during the early growing season is called and infection period.

Managing Apple Scab

The apple scab fungus survives in dead leaves on the ground and over winters there on the leaves.  A lack of spring rains can reduce its importance, but as a rule, apple scab requires yearly spray treatments.  And, ornamental crab apple trees are also hosts. As plant parts mature and the weather gets warmer, susceptibility to this disease decreases, usually in late June, but pinpoint scab can occur during extended periods of moisture during summer.  The main objective in scab management is the reduction or prevention of primary infections in spring. Extensive primary infections result in poor fruit set and make scab control during the season more difficult. If primary infections are successfully controlled, secondary infections will not be serious. The key to success in scab control is exact timing and full spray coverage. Wet periods, temperature, and relative humidity are important factors. Because scab control often is part of a combination treatment aimed at other diseases and insect control, choice of materials and timing are also extremely important.

 

How Can an Infection Period be Determined?

Apple scab infection periods can be predicted based on temperature and moisture (leaf wetness)  conditions. The Mills Table below, derived from research by Mills and La Plante, gives hours needed at various temperatures under constantly wet conditions for primary spores (ascospores) to cause infection in spring. This system for forecasting scab and timing sprays has been validated for apple-growing regions in the Midwest.

How to use the table: Figure the average temperature for the rain period by adding the maximum and the minimum temperatures and dividing by 2. If wet periods are intermittent, total their duration until there is a period of at least 6 hours of continuous dryness. You will need a wetness recorder to do this efficiently. If the dry period is sunny, and drying is quick and thorough, it is assumed that 6 hours after the trees have dried, the danger is passed. If drying is slow, and humidity remains high, then the 6-hour dry period is extended by a safety margin of 3 to 4 hours. 

 

To Spray or Not To Spray

 

Monitoring for apple scab can be quite complicated for the home grower.  But there is an alternative.  Unless wetness periods are being monitored as outlined in the section above, you can simply apply protective or eradicant fungicides at regular intervals beginning with green tip. Spraying should be done every 7 to 10 days, depending on the number of rain events between sprays.  If there are no rain events between sprays, a single protectant spray will last at least 10 days but not more than 14 days, based on the product's label directions.  You will need to make sure that your trees and fruit are protected prior to any rain event if you are going to use only a protectant.  A good protectant is Captan or Mancozeb.  But, a protectant can lose its effectivness after 2" of rain, so you also want to keep an eradicant on hand like a myclobutanil, which is available as Spectracide Immunox. A protectant like Captan has to be applied prior to a rain event.  If no protection is available during the wetting event, then only an eradicant like Immunox can be applied that has a reach back of at least 48 hours.  That means that it can still have an effect on the scab pathogen for up to 48 hours after a wetting event. A good option is to actually use both a protectant and an erdicant at the same time, like Captan mixed with Immunox, which will give you both protection and eradicant action after a wetting event. Be sure to monitor wetness periods throughout the spring to insure that trees are always adequately protected.  

More About Fungicides

 

Fungicides can be contact fungicides or penetrant fungicides and non-systemic, locally systemic or systemic. Mobility describes fungicide movement after it is applied to a plant. To understand differences in mobility, it’s important to know the difference between absorption and adsorption. 

Fungicides that can be taken up by the plant are absorbed. Fungicides that adhere in an extremely thin layer to plant surfaces are adsorbed. Because fungicides are either adsorbed or absorbed, they have two basic forms of mobility: contact and penetrant.  Regardless of the type of mobility that a fungicide possesses, no fungicide is effective after the development of visible disease symptoms. For that reason, timely fungicide application before establishment of the disease is important for optimal disease management.

Contact fungicides are adsorbed and considered non-systemic. They are susceptible to being washed away by rain  or irrigation, and most (but not all) do not protect parts that grow and develop after the product is applied.  Captan is one such contact fungicide.

Penetrant  fungicides are absorbed, so they move into plant tissues, and penetrate beyond the cuticle and into the treated leaf tissue itself.  There are various kinds of penetrants, characterized by their ability to spread when absorbed by the plant.  They can be locally systemic, penetrating leaf tissue only or systemic, moving beyond the leaf tissue. Systemic fungicides can be further subdivided based on the direction and degree of movement once they have been absorbed and translocated inside the plant. Immunox is a penetrant that is xylem mobile, therefor, not totally systemic or amphimobile.

©Janna Beckerman, Department of Botany and Plant Pathology, Purdue University

Xylem-mobile fungicides (also called acropetal penetrants ) move upward from the point of entry through the plant’s xylem.

Amphimobile  fungicides (also called true systemic penetrants) move throughout the plant through its xylemand phloem.

Locally systemic fungicides have limited translocation from the application site

Translaminar fungicides are absorbed by leaves and can move through the leaf to the opposite surface they contact, but are not truly systemic and do not move throughout the plant.  

In summary, systemic fungicides work by becoming absorbed into the plant tissues and protecting the plant from fungal diseases as well as ridding the plant of any existing diseases. Some systemic fungicides are locally systemic, meaning that the chemicals aren't transmitted very far from the application site on the plant.  Other systemic fungicides are applied to and absorbed up through the roots, moving throughout the rest of the plant. Eradicant fungicides can have systemic action, depending on which chemistry is chosen. Some are translocated within the host tissue and are able to kill the scab fungus up to a certain length of time after infection occurs. This is called the kickback or reachback period. Because kickback periods may change, always check the label for the most recent information. Kickback is calculated from the beginning of an infection period, as determined by the Mills and La Plante table. 

Rainwise Weather Station and NEWA

On April 25, 2018, I installed and set up our new Rainwise weather station. With this new station, we are now members of NEWA. The Network for Environment and Weather Applications (NEWA) delivers weather data from weather stations located on farms through the Internet at Cornell University and automatically calculates and displays weather data summaries, crop production tools, and integrated pest management (IPM) forecasts. NEWA tools promote precision IPM and crop production practices. 

NEWA was formed in 1995 and continues to evolve with advances in pest and weather forecasting. It is operated at Cornell University by the New York State IPM Program and the Northeast Regional Climate Center (NRCC)
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Weather data is radio transmitted from the weather station to the internet and then uploaded into NEWA. Climate data is archived in NEWA and run through quality control routines prior to calculating and displaying weather summaries and forecast tools for precision agriculture.

Degree days (DD) with base temperatures for different crops, insects and plant diseases, including growing degree days (GDD), are calculated and displayed. There are over 20 weather-based IPM forecast tools in NEWA. More tools are added when developed and validated.

In addition, NEWA links to climate, weather and IPM forecast products developed by other groups and researchers (National Oceanic and Atmospheric Administration, NRCC, National Weather Service, ipmPIPE, the North American Plant Disease Forecast Center, etc.).

To access our weather data at Royal Oak Farm orchard, simply bookmark this link: https://www.rainwise.net/weather/0090C2FF5987.  You will be able to access temperature, wind  direction and speed, humidity, dewpoint, pressure, isolation, precipitation, leaf wetness, growing degree days, sunrise and sunset times, the weekly forecast and radar..

Spring Finally Arrived and It's Spray Time

It's time to apply a dormant/silver tip oil and copper spray for your fruit trees! The apple trees are at silver tip in the orchard here at Royal Oak Farm Orchard in northern Illinois and that is a sure sign that spring has arrived!! It also indicates that just as soon as the nights stay above freezing, it will be time to do a dormant/silver tip oil and copper spray. The oil (mineral oil) is sprayed for mites, scale and aphids because spring is the time to cover those eggs at the base of the buds before they begin to hatch.  The oil smothers the eggs and they suffocate before hatching.  Below you can see aphid eggs that were laid last fall.

Copper is also sprayed at this time for control of fire blight and to aid in the suppression of apple scab pathogens, both being severe diseases that can destroy a crop as well as the trees. We also have to be aware of the spring critical temperatures as the buds progress in development. Each spring I post the spring critical temperatures chart from Utah State to help you determine at what stage your fruit trees may be at as spring progresses.

Ever wonder how the fruit trees know when it's time to come out of dormancy? Well, the trees won't come out of dormancy until they have endured a certain amount of time with temperatures between 32 and 45 degrees Fahrenheit. Once the number of chill hours they need is achieved and temperatures warm in the spring, the trees come out of dormancy and resume their normal growth. The number of hours required at cooler temperatures is known as the chill requirement or chill hours. As of this afternoon we had accumulated approx. 751 chill hours from October 1 of last fall. Most apple varieties require 400-1000 chill hours, so most of the trees in our area have met their requirement and will come out of dormancy just as soon as temperatures warm. Growth resumption can be predicted by tracking what we call growth units. Growth units are the number of degree hours above 41 F. For example, if the temperature averages 51 F for and hour, then 10 degree units are accumulated. Bud break initiates after approx 3710 F growth units accumulate, and progresses depending on the temperature. We do our dormant oil and copper spray generally around April 10. The best time to spray is at silver tip....when the buds have that silvery/gray tinted fuzz on them. You can use the chart below to determine the growth stage your trees may be at.

As I mentioned earlier, now is the time to do our fire blight copper spray and our horticultural oil spray.  We want to get the copper on the trees before they reach full 1/4” green and the horticultural oil can be sprayed at the same time in a tank mix or done as a dormant, silver tip,green tip, or 1/4” green spray.  In other words, your oil can be sprayed at any time from silver tip through 1/4” green providing you are using a mineral oil based product such as Superior Oil 70sec or an off the shelf Horticultural Oil such as Bonide’s All Seasons Horticultural Spray Oil.  Your copper spray should be done before the trees reach 1/4” green to avoid any phytotoxicity issues.  For your copper spray you can also use an off the shelf brand such as Bonide Copper Fungicide RTU (Ready to Use). Since the Bonide Copper Fungicide contains Copper Octanoate (Copper Soap), it is safer to use on your trees than heavier metallic copper compounds that could have phytotoxicity issues after trees have green tipped. Both of these products should be available at your local hardware store or garden center of from Amazon.com.  We also have supplies at our Nursery at Royal Oak Farm Orchard.

A dormant oil and copper spray should not be done until we get at least a 24 hour period that is above freezing at night. The oil cannot freeze on the trees, but it pretty much dries within about 24 hours. Once dry, there is no chance of it freezing.  We usually get at least one 24 hour period above freezing at night before the trees get to 1/4" green. 

This “window of opportunity” for dormant sprays for fruit trees depends on the bud stage of your target fruit tree. You can follow these guidelines:

Apples: swollen bud to 1/4” green
Pears: swollen bud to cluster bud
Peaches/Nectarines: swollen bud to pre-bloom
Apricot: before bloom

When applying, spray trees just until they are dripping to get good application on all the stems and crevices at the buds. If you are using horticultural oil alone, use a rate of 2% (mixed in water) for best results or follow your chosen product’s label rate.  For situations where aphids have been real problems in the past, consider adding an insecticide (such as acetamiprid, etc.) to 1.5 - 2% oil or use one of the Bonide RTU (ready to use) pre-mixes for insect pests.

Time to Start Thinking About Pest Control

Many homeowners enjoy raising their own fruit, but anyone who has attempted to grow fruit in their backyard knows that fruit crops are attacked by a wide variety of insect and disease pests and prone to environmental damage, especially in the Midwest.  For your Home Orchard, we recommend the use of an entirely new approach to managing pests called Biointensive Integrated Pest management (Bio-IPM) to minimize the insect and disease damage to your fruit. Biointensive IPM utilizes a systems approach to pest management based on an understanding of pest ecology and tree physiology.  It begins with steps to accurately diagnose the nature and source of pest problems, and then relies on a range of preventive tactics and biological controls to keep pest populations within acceptable limits.  The preventative tactics include a combination of ecological, biological, natural, and cultural controls to keep applications of chemical and organic controls to a minimum. The goal is only to spray as a last resort for the control of pest and disease and to only use the most environmentally friendly materials.

In the northern regions of Illinois and in most apple growing regions east of the Mississippi, there are four main pests of apple trees.  Those four main pests are plum curculio, codling moth, apple maggot, fire blight and apple scab. To learn more about these pests and how to control them, you may want to download the available pest fact sheets  and the "Managing Pests in Home Fruit Plantings" guide from Purdue University available here or on our web site. This publication provides homeowners with the information they need to produce an acceptable amount of quality fruit (apples, pears, peaches, plums, cherries, grapes, strawberries, raspberries, blackberries, currants, gooseberries).

  

Often there are enough beneficials (insects that prey upon other insects) to control the pest(s) in your orchard without spraying.  On other occasions you might use traps to catch pest species as they enter your orchard, or determine from the traps that there are too few of the pest to cause serious damage to your trees or fruit. But if you do have to control insect pests, there are many new all natural products on the market today that can be a 100% ecological solution.  One of those products is spinosad. Spinosad is a natural substance made by a soil bacterium that can be toxic to insects. It is a mixture of two abcterium called spinosyn A and spinosyn D. It is can be used to control a wide variety of insect pests. But  always, as a last resort, reduced- risk pesticides may need to be used if other tactics have not been adequately effective, and with care to minimize risks. 



 As the growing season approaches, now is the  time to determine what pest problems you had last season, or may have this season, and what the best approach is to handle those problems.  As the various stages of tree growth develop, so does the growth of pests.   The "Managing Pests in Home Fruit Plantings" guide will provide for you a chart of the growth cycles of the tree and a spray guide chart to let you know what pests are prevalent during those growth stages.  The "Spray Guide" will give you a list of environmentally friendly products you might need to use against those pests as a last resort.  Remember, spray chemicals is a last resort, but is also necessary if bio-controls are not working on your pests.


 
The benefits of implementing biointensive IPM include reduced chemical input costs, reduced environmental impacts, and more effective and sustainable pest management.  An ecology-based IPM has the potential of decreasing inputs of natural chemicals and synthetic chemicals - all of which are energy intensive and increasingly costly in terms of financial and environmental impact.  All these efforts make it possible for you to apply chemical controls only a few times each season when they are truly required. And we can recommend the use the most environmentally friendly materials available in our Nursery Center.      

I hope you find this post useful!  As always, if you have any questions contact me anytime via comments or through our web site!