Apple Scab Has Arrived to Northern Illinois

Based on our NEWA (Network for Environment and Weather Applications) Apple Scab Model, we have had multiple scab infection events beginning March 9 with spore maturity being very low (see chart below). But, the scab season has officially begun!  With no leaf tissue showing prior to green tip, any previous infection period would have no immediate effect on the leaf tissue, but we now know that there are spores out there.  Since we had our copper spray put on right at green tip the evening of April 10, we were protected from the infection once the spores began to mature.  With spore maturity and discharge being very low, the risk is also very low, but we still need to be protected because any high temperature day can cause the spores to mature very quickly. 

Ascospores mature as spring progresses with a few ascospores usually maturing by bud break (green tip). The proportion of ascospores maturing progresses slowly until about the tight cluster stage of blossom development. From tight cluster through bloom the percentage of mature ascospores rapidly increases with most ascospores matured by the end of bloom. 

Unusual weather conditions may contribute to significant ascospore discharges earlier than or later than the model predicts. 

Mature ascospores begin to discharge into the air within 30 minutes during periods of rain. When rainfall begins at night, discharge may be delayed until daybreak. Ascospore discharge usually peaks from pink through bloom, and nearly all ascospores have been discharged within 1 or 2 weeks after petal fall.

Copper has been shown to be a protectant against scab on the tissue that has been covered, but not on any new tissue that would have grown since that copper spray was put on.   If no copper spray has been put on your trees as of today, and you have not put any other protectant on like Captan or Mancozeb, you still have time to get an eradicant fungicide put on your trees in the form of Immunox, which has a 72 to 96 hour reach back ability.  That means that you have up to 72 to 96 hours to get a spray on which will still be effective against any scab infection event 72 to 96 hours previous. Immunox is a xylum mobile fungicide which means when applied to leaves it will move throughout the leaves it was deposited on but will not move out of that leaf.  

For the home grower who may not have a weather station or scab model to help predict the apple scab infection periods, monitoring for apple scab can be quite complicated.  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, which I mention above.  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 72 to 96 hours as I have stated above.  That means that it can still have an effect on the scab pathogen for up to 96 hours after a wetting event, although it is a good practice to use the shorter 72 hours.. 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. 

 But, 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.

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This publication contains pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these  recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned.  The author assumes no liability resulting from the use of these recommendations.

 

Apple Scab Season is Upon Us

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.  In some cases, due to our uncertain weather pattern, some trees are still at silver tip and others are near 1/2" green, but as long as any leaf tissue is exposed, there is a chance for primary scab to set in.  Apple scab is the most prevalent and most damaging disease to apples we have in the Midwest and pretty much all apple growing locations east of the Rocky Mountains.  At this point, scab sprays should be applied according to the Apple Spray Guide found in  "Managing Pests in Home Fruit Plantings".  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.
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This publication contains pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these  recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned.  The author assumes no liability resulting from the use of these recommendations.

Time to Start Spraying to Control Pests

If you have not done so already, it's time to begin thinking about a dormant oil and copper spray for your fruit trees! The apple trees are at silver tip and approaching green tip in the orchard here at Royal Oak Farm Orchard in northern Illinois. We may green tip fully this week with the daytime temperatures, but those night projections below freezing will keep the tree growth to a minimum for the remainder of the week temperatures.  It also indicates that just as soon as the nights stay above freezing, the trees will pop and we will be moving into apple scab season.  So it is not too late to do a dormant oil and copper spray if you have not already.  We completed our spray on April 18.   

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.  For those who may have ,missed my last post, here are the charts again.
 

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.  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.

As I mentioned earlier, now is the time to do our fire blight/scab 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).  Both of these products should be available at your local hardware store or garden center of from Amazon.com.

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.

For get our Growing Guide on maintaining your trees and Resources for the Home Orchard Grower of the above charts, visit our Nursery Growing Guide web page!

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This publication contains pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these  recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned.  The author assumes no liability resulting from the use of these recommendations.

Critical Temperatures for Freeze or Frost Damage to Apple Trees

With the predicted colder night temperatures we are sure to see, it's always a good idea to revisit the critical temperatures that can cause frost or freeze damage to fruit trees, specifically apple trees.  We are currently at silver tip to green tip here in northern Illinois with a forecast of night temperatures in the low 20's next week. Fortunately, our trees haven't progressed to far this spring, but we could see green tip before next week.   This spring has marked another unprecedented weather pattern that raised our temperatures in late March and early April and is dropping our temperatures going into mid-April to way below normal.  The early warm temperatures accelerated bud swell, and the lower temperatures that are anticipated present the threat of frost or freeze damage to new green tip leaf tissue.  It seems that each spring since about 2012, we have been on the verge of critical temperatures for frost or freeze damage with our fruit trees. 

Dark brown centers and signed appearance of the petals
indicate that both king and side blooms were killed in a
freeze the morning this picture was taken.  (Photo credit:
Mark Longstroth, MSU Extension)

As the trees develop in the spring and buds start to swell, they lose the ability to withstand the cold winter temperatures that they could withstand in dormancy during the cold winter months. The young, actively growing tissue can then be damaged or even killed. Swollen fruit buds can better withstand temperatures in the teens without any damage. As the buds open, temperatures in the low 20s can cause harm, but sometimes leave other buds undamaged.  As growth moves from green tip to 1/4” green to 1/2” green to tight cluster to pink in apple trees, temperatures in the upper 20s can cause considerable harm to an early blooming tree. Near bloom, the range between slight and severe damage can be very small. Freezing temperatures of 28 degrees F. will result in about a 10 percent loss and 24 F will result in a 90 percent loss, as indicated by the charts down below.

The dark brown center of this apple flower  indicates it was
killed by a freeze. (Photo credit: Mark Longstroth, MSU Extension)

In a radiation freeze with clear, calm conditions, fruit at higher elevations or in the tops of trees will be less damaged than those at lower elevations, since colder air is more dense than warmer air and sinks to ground level, pushing the warmer air up. The percent of flowers killed in a frost may or may not relate directly to lost yield later in the season. With large-fruited fruits such as apples, peaches, plums and pears, the loss of 50 percent of the flower may not be devastating since we may only want a small percentage of the flowers to become fruit, meaning that fruit thinning may be totally unnecessary.  So the stage of bud and bloom development determines how susceptible any given fruit crop is when freezes occur.  For more information on what those critical temperatures are that can cause freeze damage to trees during development, I have added two charts on the Critical Temperatures For Frost Damage on Fruit Trees from Utah State University below that you can download by clicking on either chart below.

(Click on the photos to download the chart in PDF format.)

 

Given the weather patterns we have experienced so far this spring a spring frost could still be possible.  Once the fruit has set, then the critical temperatures that can damage the fruit become lower.   We will need to constantly assess the stage of development our trees are at over the next weeks and their susceptibility to possible freeze injury.

If we continue in this spell of colder weather, apple trees will continue to develop more slowly, but once they begin showing tight cluster, pink and bloom, the critical temperature rises from the low 20’s to the high 20s, to levels just below freezing at bloom time, which is the most critical time to get frost damage.