June 2, AppleTalk Call Summary

AppleTalk Conference Call Summary
Tuesday, June 2, 2015, 8:00 – 9:00 AM
Guest speaker: Amaya Atucha, University of Wisconsin, Assistant Professor/ Fruit Crop Specialist
Presenter: John Aue, Threshold IPM
Moderator: Peter Werts, IPM Institute of North America; questions or comments, pwerts@ipminstitute.org

June 2nd Call download: Click Here

Apple scab 1:55

Most areas had a long infection period on May 24 through May 28, 2015.  Scout for scab lesions seven to ten days following an infection by scanning the undersides of leaves of scab-susceptible varieties or in blocks with a history of scab, for light-gray lesions.  If lesions are found, look for trends on terminal shoots and fruit clusters to help determine when an infection took place.  Active lesions will continue to infect new-leaf tissue through conidial infections (secondary infections).  Focus control measures on problem blocks for the next six weeks or until the leaf tissue has matured.  If no lesions are found proceed with reducing fungicides applications for the remainder of the season.

Rainfall totals within the last week range from a half-inch to five inches.  Protectant fungicides, e.g., captan and EBDCs, lose half of their efficacy after the first inch of rain and after two inches of rain most to all of the protection is lost.  The intensity of rainfall influences the wash off rate, e.g., heavy rainfall following an application can remove material more quickly than a steady, light rain over the course of a week.

Assess the amount of new leaf and fruit tissue and rainfall from last application to determine if reapplication is necessary.  Use the weather forecast to time application; dry weather will allow more time between a reapplication.  It is recommended to apply a full rate of Captan with the last cover spray targeting primary scab for added protection against black rot infections.

Plum curculio and tarnished plant bug 7:50

Degree-day accumulations following petal fall is on track regardless of the fluctuating temperatures.  The absence of an extended warm period, i.e., nighttime temperatures greater than 60°F, following petal fall does not offer assurance that plum curculio (PC) have migrated into orchards.  If an insecticide has been applied for PC or tarnished plant bug (TPB) assess the effectiveness by scouting for damage, while evaluating thinning results.  An approximate diagnosis of the age of PC oviposition scars or TPB feeding injury can be given by observing the size and depth of the injury.  Evidence that may suggest damage occurred more than a week ago is the depth of the cone caused by TPB or if the PC oviposition scar has begun to form a dimple.  Differentiating between new and old damage will provide insight if PC or TPB is active in the orchard and offers assurance that the insecticide targeting the pest was effective.

Codling moth 11:10

Codling moth (CM) flights this season have been variable; with very minimal flights occurring in some regions.  This winter there were no weather conditions which negatively impacted populations CM.  To date, many orchards have not set a concrete biofix.  It is recommended to increase the number of CM pheromone traps, if catches remain low.  This may help capture moths which fly in localized areas of the orchard.  Locate CM traps at a density of one per 2.5 acres or one per five acres, where blocks are uniform in size, shape and topography.

Use trap catches to assign a biofix and to determine when to apply a larvacide.  If the flight is lite the first larvicide application timing can be delayed to 350 DD, base 50°F, from biofix.  For a heavy flight, i.e., weekly trap captures above threshold, apply a larvacide at 250 DD.  Track degree-days from the beginning of the first flight, regardless of intensity, to differentiate first generation from second generation; second generation flights begin 1000 DD after the beginning of first generation.  Change insecticide mode-of-action for second generation CM.

Q & A with guest speaker Amaya Atucha, University of Wisconsin 16:50

Tree Stress: It’s believed three stressors are attributed to cause the massive winter injury of spring 2014.

  1. June and July 2012 were extremely hot and dry, a number of days above 100°F. 
  2. Early bud-break in March 2012 was followed by freeze damage to flowers and fruitlets – resulting in crop losses of 40% – 90%.  While not a stressor, this light fruitset resulted in many growers not irrigating established trees.  This also set growers up for a very large crop in 2013.
  3. Winter of 2013-2014 was longer and significantly colder than normal, followed by a slow, cool spring.

Questions flowing from this scenario:

How do these three types of stress, (heat & water, heavy crop, multiple sub-zero temps) differ, if at all, in terms of tree physiology, recovery time, altering disease susceptibility?

Heat, water and a heavy crop load are all factors that will limit photosynthesis and will decrease carbohydrate production.  During dormancy carbohydrates are stored with water, in plant tissue and are valuable for cold hardiness and growth in the spring.  A tree with a depleted carbohydrate reserve is more prone to winter injury due to the increased space available for water to freeze in the plant tissue.  A heavy crop load can be one of the greatest stressors, since the this depletes carbohydrate reserves at a greater rate.  Maintaining a balanced crop is important for the following season’s yield potential and will also improve cold hardiness and growth during the spring.

The amount of available nitrogen (N), is another factor that affects cold hardiness.  An excess supply of N during a lite crop year will promote vegetative growth and result in an abundance of water stored in plant tissue during dormancy.  In a heavy crop year, where carbohydrate reserves are depleted, N and potassium can be applied immediately after harvest, when the tree is still actively photosynthesizing.  These nutrients can be applied at a lower rate during years with a lite to normal crop.

The application rates of nutrients are dependent on multiple factors, e.g., age of tree, soil organic matter, variety and vigor.  Generally, 5-10% of the total nitrogen applied during the growing season can be applied immediately after harvest.  A foliar application is the recommended method to apply nitrogen post-harvest, all of the nitrogen will be stored in the buds for the following spring.  Nitrogen and dolomite lime can be applied be applied as a ground application to adjust the soil pH.

In years with a lite crop, calcium deficiencies in the fruit are often more widespread.  Excessive vegetative growth increases the transportation of water and movement of calcium to the shoots and leaves, rather than to the crop.  It is recommended to apply more calcium in these years to reduce the incidence of bitter pit.  Apogee (prohexadione calcium) can also be applied to reduce vegetative growth.

Severe stress periods in woodland tree species are often visible for decades – can the recovery time-frames for tree fruit extend essentially for the life of the tree?  Is there any research into how various rootstocks may respond?

The severity of damage influences the recovery of the tree.  Minor damage, e.g., damage to fruit or vegetative buds, can be corrected with pruning and will allow the tree to recover within one to two years.  Major damage, e.g., damage to scaffold or roots, will cause the tree to collapse and will take a very longtime to heal.  Major damage resulting from winter injury is similar to if the tree was girdled.

Currently, there is not enough research to explain the healing process of root systems or the cold hardiness of the roots of the rootstock.  The smaller root systems of dwarfing trees are more confined to the irrigated volume of soil and are more sensitive to stress.  The roots of larger trees can explore further reaches of the soil for water and are not restricted by irrigation.  Generally, irrigated trees are less stressed during the growing season and can tolerate colder temperatures due to the larger quantity of carbohydrates reserved in the roots.  Soil with more moisture going into winter will stay warmer while the tree enters dormancy and will allow the tree more time to acclimate.  Dry soil will get colder faster and may not allow the trees to acclimate.

How much stress can a healthy trees endure before symptoms develop?

Visual signs of stress are the key indicators of a tree’s health.  If visual symptoms are not exhibited the health of the tree may not be assessed.

Fertilization: Most growers have begun replanting for above tree losses.  Many want vigorous growth, especially on trellis plantings.  Please discuss the various N sources to address volatility, availability, and maximum application rates.

Nitrogen fertilizers can be divided into two groups: ammonium-nitrogen and nitrate-nitrogen.  Ammonium-nitrogen is a positively charged compound and is readily absorbed onto soil particles, reducing loss through leaching.  Leaching is the process of water dissolving pollutants as it travels through the soil profile.  Soil type, precipitation, chemical properties of the pollutant, application method and rate and style of planting are variables that can affect the leaching potential of a site.  Although the leaching potential is low, ammonium can be readily lost through volatilization.  Volatilization is the process of converting a chemical substance from a liquid or solid state into a gaseous or vapor state that can easily leave the system.  For example, when urea is applied to wet, warm, basic soils (pH greater than 7) a third of the nitrogen can be lost to volatilization.  Soils with a low cation exchange capacity, e.g., sandy soils with low organic matter, lose more to volatilization than soils with a higher cation exchange capacity, e.g., clay soils with high organic matter.  To reduce volatilization it is recommended to incorporate the fertilizer into the system through cultivation, fertigation or applying as a concentrate band application within the dripline of the tree.

Nitrate-nitrogen, e.g., calcium nitrate, is a negatively charged compound and is susceptible to denitrification.  Denitrification is a process that is facilitated by soil microbes that convert nitrogen to nitrogen dioxide.  More nitrogen is lost to denitrification in soils with a high moisture content.  If applying calcium nitrate use caution to not apply to saturated soils or before irrigation or heavy rain events

Note: Both ammonium-nitrogen or nitrate-nitrogen can be lost in saturated soils.

When fertilizing non-bearing for maximum shoot extension, discuss known trade-offs, if any (such as insect or disease susceptibility, strength, winter hardiness, etc.)

Nitrogen should be delivered to newly planted trees in small doses, i.e., 1/4 lb. of calcium nitrate, and applied at about two weeks after planting as a doughnut-shaped band around each tree.  Frequent low doses of nitrogen fertilizer delivered at least twice weekly through the trickle system (fertigation) for the first 12 weeks of the season will greatly improve tree growth during the first two years to speed development of the canopy.

Please lay out your preferred timetable for fertilization, growth, senescence, that maximizes growth needs as well as achieving winter hardiness.

Nutrient application rates and schedules are extremely site dependent.  It is recommended to acquire foliar and soil nutrient analyses before applying fertilizers.

Visual indicators of excessive nitrogen on a bearing tree may be excessive growth, e.g., greater than 18 inches, a nitrogen deficiency may be shown by growth less than eight inches.  In general a young apple tree should have between 2.1-2.4% nitrogen.  Nitrogen requirements are lower on bearing trees compared to young, non-bearing trees.

Are there “red flags” going into fall that can indicate increased probability of winter injury?

Trees should have their terminal buds set by mid-August.