Article written by Madeline Wimmer, UMN Extension Fruit Production Educator.
Heat advisory for Minnesota growers
Apples
Identifying codling moth damage: Deep entry
When should trunk guards be removed from apple trees?
Check trellis wires, clips, and bands during the growing season (high density)
Grapes
Insect gall identification
Kiwiberries
Article: Kiwiberry production in Minnesota: What's the potential?
Heat advisory for Minnesota growers
It’s a hot week in many parts of the state. Try your best to stay cool, work during the coolest parts of the day, take breaks, and know how to recognize signs of heat exhaustion for yourself and other workers.
To learn more about protecting yourself and others from extreme heat and air quality issues, visit our UMN Extension web page:
Develop a heat and air quality safety plan for your farm workers.
Identifying codling moth damage: Deep entry
Images: An Gravenstein apple showing signs of damage classic to codling moth (Cydia pomonella). A: frass at the apple calyx end is characteristic of codling moth larvae. B: When the frass is removed, a hole is revealed. C: the apple damage was localized to the apple core. Photos taken at an orchard in Sonoma County, California (Zone 9b).
Codling moths (Cydia pomonella) damage apples when adult moths lay their eggs near apple fruitlets. The larvae, which are light pink with a dark head, emerge from the eggs and can cause damage to fruit. When the larva tries to enter a fruit and fails, its attempt can lead to a superficial, small wound known as a codling moth “sting,” which is often tolerated for apples harvested as “seconds,” which are not top-selling in terms of quality, but are still suitable for consumption.
More severe damage can happen when the larva successfully enters the fruit and begins to feed and grow. This leads to a burrow within the fruit that usually passes through the apple core and seeds, and leaves frass in its trail, which can plug up the entry hole (commonly at the calyx end).
This feeding pattern differentiates codling moths from other moths, or lepidopteran pests like the oriental fruit moth (Grapholita molesta), which tend to move in more random patterns. It also is different from apple maggot, which often leads to deformed fruits and smaller trails within the apple flesh.
After codling moth larvae are fully grown, they exit the fruit and generally pupate for 2–3 weeks. After that point, a second generation of moths emerge and can be monitored similarly to the first generation. In warmer climates with longer growing seasons, there can be three generations of codling moth.
Depending on the growing region and how many degree days have accumulated, this is the time of year when second generation codling moth can begin to emerge. If you were tracking populations earlier in the growing season, this is a good time to start checking traps that have been refreshed (replace sticky liners as needed and between flight generations). Remember to replace codling moth pheromone for traps every 4–6 weeks, or refer to the specific product packaging.
For those who have weather stations connected to NEWA, you can enter the date for your first sustained trap catch of the first adult generational flight for pest stage estimates and guidance specific to your site.
To learn more about codling moth management, check out the UMN Fruit and Veg article written earlier this season below:
Article: Codling moth: A common insect pest to manage in Minnesota.
When should trunk guards be removed from apple trees?
Images: Two pictures showing tree guards on older trees. One is installed on a central leader trained tree (left) and the other on high-density trained tree (right).
Trunk guards are helpful for apple tree establishment, as they protect against damage from wildlife and herbicide sprays. But when is the appropriate time to remove them, and what are the consequences of leaving them on too long?
Before we dive into this question, it’s important to recognize that there are a wide variety of guards that can be placed on a tree trunk. They can vary in color, which alters the amount of heat they absorb or reflect, along with the material composition, thickness, and design. Some guards are very solid, while others allow for more airflow, which can lower the humidity around the tree trunk. Some metal guards are sturdy enough to push into the ground to help with vole damage.
A metal tree guard can be pushed into the ground easier than some plastic guards. Photo taken at Third Leaf Farm located in Kent County, MI.
Depending on the type of guard used, how much air flow it allows, and other properties, it may be more important to remove guards earlier than later. Prolonged use of tree guards can be problematic. For the two images shown above, the tree on the left had moss growth limited to trunk parts located within the trunk guard. This is a sign that the guard was creating a very moist environment. Other issues can develop too, like trunk injury from rubbing against and growing into the guard as the tree ages and expands.
One other consideration based on the guard color relates to heat absorption and the potential for rootstock shank or trunk injury. As black surfaces absorb more solar radiation than white, there is a question of how much this could potentially impact trunk temperatures, contributing to higher trunk temperatures and possibly put the trunk at greater risk for winter injury.
Additional factors that can contribute to when guards are removed include tree vigor, site conditions, and wildlife pressure, as guards can typically be removed once trees are no longer at risk for damage, and they should be checked periodically.
Like many choices growers face in production, the risks of keeping the guards on should be weighed against the potential risks associated with their removal, like wildlife damage on trees with less wood development. For small orchards, guards can be put on during winter, a high-risk time for wildlife damage, and taken off again in the spring.
Check bands, fasteners, and clips to prevent girdling
Images: Two apple trees trained on high density trellis systems at two different growing sites. Left: an apple tree was fastened to the training wire with a clip that the trunk is currently growing over. Right: The beginning signs of trunk girdling can be seen here from a T-band used for fastening.
During establishment, dwarf and some semi-dwarf apples benefit from securing trunks onto either training stakes or trellis wires using training supplies (while some trellis systems may only use a hanging wire or string to the top wire).
Wire clips provide a simple way to fasten trunks, but need to be checked and eventually removed or replaced to prevent trunk damage from girdling, which can be extreme at times, as seen in the above left photo where an apple tree trunk has grown around the trellis wire and clip.
Rubber T-bands are better able to stretch as the tree ages, but also need to be checked and either removed or replaced with larger bands as the trees grow. When the bands are too tight, they can rub on and wound the adjacent bark, which makes the tree vulnerable to certain pathogens.
Grapes
Insect galls
Images: Grape tumid gall maker (left photo; Vitisiella brevicauda) can create fuchsia-colored, bumpy galls that protect developing larvae. Filbert galls (right photo) are created by Ampelomyia vitiscoryloides. Both gall types very rarely cause economic damage to grapevines, but should be removed when noticed. Photo (left) was submitted by a grower from Polk County, MN, and photo (right) was submitted by a grower from Winona County, MN.
A gall is a physical growth that leads to distortion of a specific plant part. The grape phylloxera is one of the major insect pests that creates galls on grape leaves, but there are other insects that occasionally produce galls too. While they are not usually a cause for concern or need for management, it is still helpful to know what they look like.
Photo: The grape phylloxera is a common pest that impacts cold climate grape leaves. The galls are small and can be seen on the leaf underside. Most grapes can tolerate some amount of galls each season without economic impact.
Grape tumid gall maker
The grape tumid gall maker is a midge fly that lays eggs within unfolding buds and developing shoots. After the eggs hatch, the fuchsia-colored gall (left photo at the top of the article) begins to form as the larvae feed. There are a wide range of tissues where galls can be formed, including leaves, shoots, petioles, tendrils, and even flower clusters.
The damage is usually very minor as cluster infection is not guaranteed or common. If you see galls in your vineyard, removing the infected tissues and destroying them is adequate and no insecticides are needed for its management.
For vineyards that are uniquely challenged with high populations, it is still difficult to time insecticide applications because the adults only live for one day. One cultural strategy to manage the adults is to attempt to bury the pupae on the ground in the early spring with less-coarse types of mulch or dirt, which can be done near vines that had issues the previous year.
Grape filbert galls
Grape filbert galls, also known as hazelnut galls (right photo at the top of the article), are caused by the midge fly Ampelomyia vitiscoryloides. These galls look similarly bizarre—they can have a smooth or fuzzy texture—are difficult to miss, and very rarely cause economic levels of damage on grapes.
Kiwiberries
Kiwiberry production in Minnesota: What's the potential?
Images: Actinidia kolomikta (top left and right photos) has unique variegated leaves and is grown along with Actinidia arguta (bottom left and right photos) at the University of Minnesota HRC for fruit breeding.
Kiwiberries, sometimes referred to as hardy kiwis, are considered an emerging crop for commercial production in Minnesota and throughout the Upper Midwest. There is interest for growing this crop for both commercial, hobby, and backyard growers, but with this interest comes a number of considerations to determine the viability of the kiwiberry as an Upper Midwest fruit crop.
To learn more about kiwiberries and read the rest of the article, click on the link below to see the article.
Article: Kiwiberry production in Minnesota: What's the potential?
Thank you to our farm and ag professional partners for contributions to the UMN Fruit Update series. Non-credited photos in this article were either taken by Madeline Wimmer or within the UMN Extension system.
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