Madeline Wimmer - UMN Fruit Production Extension Educator
This article marks the beginning of the 2025 UMN Fruit Updates, where information is gathered and shared in real time about current happenings and production requirements for fruit crops in Minnesota and the Upper Midwest. These articles are released regularly throughout the growing season, and readers can receive notice of new articles by subscribing to the UMN Fruit and Veg News newsletter email alerts.
While apples and grapes are a large focus of this series, we’ll also include updates on other Minnesota-grown fruit crops—such as strawberries, raspberries, pears, plums, blueberries, honeyberries, and currants—based on field conditions and emerging needs throughout the season.
Fruit management topics in this series include: tracking plant growth and fruit ripening stages; horticultural management options to improve fruit quality; integrated pest awareness sections that identify and describe various insect and disease-causing organisms; information about pollinators and other beneficial insects; and other production topics to help you succeed in growing fruit crops in Minnesota and surrounding regions.
This fruit update contains information about…
Apples
Images: Apple trees showing varying degrees of the developmental phase known as green tip (lower right) and nearing ½ inch green (top and lower left images). Green tip is a marker, or biofix date, that can be used as a reference to estimate when all spores have been released from fruiting bodies of the apple scab pathogen, Venturia inaequalis. Photos taken at Fall Harvest Orchard on 04/18/25 (Delano, MN, Zone 4b).
Fruit crop growth stages are more than just a way to track development—they also help indicate when to start watching for things like pest activity. The 50% green tip date, for example, can be used as a biofix date, or a biological marker, to help estimate when other biological events might occur. In the case of apple scab, the 50% green tip date is used to determine when all overwintering primary spores (i.e., ascospores) have been released from fruiting bodies of the apple scab pathogen, Venturia inaequalis (see section below to learn more about apple scab). This year, we’ll track apple scab spore development using the Network for Environment and Weather Applications (NEWA) by entering the UMN HRC’s green tip date, where ~50% of McIntosh apples were at green tip: 04/16/2025. As of today, most apples throughout the state are somewhere between silver tip and tight cluster, depending on the region and apple variety. We’ll continue tracking apple growth stages throughout the year.
Note: The NEWA website as of now requires manual entry of green tip date.
Images: Apple scab can cause a wide range of damage on various apple types based on their inherent resistance, the amount of spores present at spring, along with weather conditions and orchard management programs. Damage can be seen in the above pictures as late-season apple scab lesions and chlorosis (yellowing) on a crabapple leaf (left), and minor apple scab infection on a ripe apple right.
Apple scab is considered to be the most common disease of apples in Minnesota. It can be mild to severe in how it impacts production, depending on how much action is taken to manage and prevent infections. The disease-causing organism is the fungal pathogen Venturia inaequalis, which is a part of the ascomycota: a classification of fungi that produce tiny, spore-producing fruiting bodies, similar to mushrooms. When present, these fruiting bodies can overwinter primarily on leaves, which then provide spore inoculum in the spring that can infect new leaves and flowers when conditions are wet and warm enough.
The stage at which these primary spores are being released from overwintering fruiting bodies is referred to as primary infection and causes lesions on leaves and fruits where new spores, known as conidia, begin to form and multiply. The process of primary spores being discharged can generally last until 1-2 weeks after petal fall. It can also be estimated using degree day (DD) accumulations starting at the green tip date, where most primary spores will be discharged by 400-600 DD, depending on weather conditions and other factors. The combination of the green tip date, tracking rain events, temperature, and degree days units, is how infection models like NEWA begin estimating percent primary spore, or ascospore, maturity and cumulative discharge throughout the season.
Infections that occur after this point are known as secondary infections and result from any conidia that established and multiplied based the original primary infections. Thus, early season management that eliminates primary spores from infecting apples is advantageous to keeping apple scab infections under control, especially in orchards with previous season infections. Which might be relevant this year. Last growing season, many parts of Minnesota experienced considerable amounts of rain events from spring until early summer, which impacted apple scab development in landscape settings and in orchards where the starting spore inoculum was high, or management practices were limited for whatever reason. Orchards in these circumstances may want to adhere to a more attentive management program this year.
Mowing or removing leaves of infected trees: If you think your orchard is at higher risk this year, collecting and flail mowing, or removing leaf litter is a cultural management practice that can help encourage leaf decomposition and reduce risk for apple scab infection occurrence. For mowing, it’s recommended to blow leaves into the aisle, or gather them by some other means.
Apple scab resistance: Certain apple varieties have some resistance to apple scab, which includes HoneycrispTM. Resistance is based on a single or multiple apple genes that help either stop or slow down apple scab disease development. One of UMN’s most recent apple releases, Triumph is considered more resistant to apple scab than other varieties like Honeycrisp, making it a potential option for growers with limited spray options. One way to increase apple scab resilience for new orchards, or new orchard plantings is to choose resistant apple varieties. This information can usually be found through nurseries suppliers and through fact sheets online.
When apple scab is present in an orchard, primary spore release and subsequent infections can happen after rain events and during wet conditions that have adequate humidity and warmth. Below is a table originally produced by Mills and La Plante that compares the amount of time it takes in hours for primary spores to cause an infection at a given temperature
Table originally created by Mills and La Plante. An accessible version of this table may be found on the UC IPM webpage Apple Scab.
Recognize that this table can help growers estimate the risk or likelihood of primary and secondary apple scab infection, and help guide fungicide timing of applications. However, because it assumes continuous leaf wetness, the Mills-La Plante table may not fully reflect real-world conditions, which can lead to reduced accuracy in certain situations. For the purposes of this article, it can help to understand how increases in ambient temperatures can decrease the amount of time it takes for infection to occur during wet periods.
Images: Grape flea beetles (Altica spp.) can lay their eggs on developing buds and bark, or on the underside of grapevine leaves as seen in the leftmost picture here. As adults emerge from winter, they feed on grape buds and mate. Damage to developing buds can interrupt shoot growth. Photos credited to T.L. Galvan and E.C. Burkness.
Thank you to our farm and ag professional partners for contributions to the UMN Fruit Update series. Non-credited photos in this article were taken by Madeline Wimmer.
This article marks the beginning of the 2025 UMN Fruit Updates, where information is gathered and shared in real time about current happenings and production requirements for fruit crops in Minnesota and the Upper Midwest. These articles are released regularly throughout the growing season, and readers can receive notice of new articles by subscribing to the UMN Fruit and Veg News newsletter email alerts.
While apples and grapes are a large focus of this series, we’ll also include updates on other Minnesota-grown fruit crops—such as strawberries, raspberries, pears, plums, blueberries, honeyberries, and currants—based on field conditions and emerging needs throughout the season.
Fruit management topics in this series include: tracking plant growth and fruit ripening stages; horticultural management options to improve fruit quality; integrated pest awareness sections that identify and describe various insect and disease-causing organisms; information about pollinators and other beneficial insects; and other production topics to help you succeed in growing fruit crops in Minnesota and surrounding regions.
This fruit update contains information about…
Apples
- Growth stage updates.
- Resources for apple pest management.
- Disease highlight: About apple scab and significance of early season management.
- Growth stage update and introduction to E-L chart.
- Tying grapevine canes.
- Resources for grape pest management.
- Insect pest highlight: About grape flea beetles.
Apples
Growth stage update
Green Tip - 1/2 inch Green
This week’s growth stage update comes from apples located west of the twin cities, near the University of Minnesota Horticulture Research Center (UMN HRC), where many apple varieties grew into the green tip stage last week and are either approaching ½ inch green or tight cluster this week. Green tip is the stage that follows the silver tip phase and is the point where leaf emergence begins. In the above photo collage, the top and lower left photos are in late green tip and approaching the ½ inch green phase.Fruit crop growth stages are more than just a way to track development—they also help indicate when to start watching for things like pest activity. The 50% green tip date, for example, can be used as a biofix date, or a biological marker, to help estimate when other biological events might occur. In the case of apple scab, the 50% green tip date is used to determine when all overwintering primary spores (i.e., ascospores) have been released from fruiting bodies of the apple scab pathogen, Venturia inaequalis (see section below to learn more about apple scab). This year, we’ll track apple scab spore development using the Network for Environment and Weather Applications (NEWA) by entering the UMN HRC’s green tip date, where ~50% of McIntosh apples were at green tip: 04/16/2025. As of today, most apples throughout the state are somewhere between silver tip and tight cluster, depending on the region and apple variety. We’ll continue tracking apple growth stages throughout the year.
Note: The NEWA website as of now requires manual entry of green tip date.
Resources for apple pest management:
Throughout the Fruit Update series, we’ll reference a number of different pests, including insect, disease-causing organisms, and competing plants to help you maximize knowledge and resources to support your pest management. This week we released an article with various resources to equip you with the knowledge and tools for apple pest management this season.Apple Scab: About and significance of early season management
Apple scab is considered to be the most common disease of apples in Minnesota. It can be mild to severe in how it impacts production, depending on how much action is taken to manage and prevent infections. The disease-causing organism is the fungal pathogen Venturia inaequalis, which is a part of the ascomycota: a classification of fungi that produce tiny, spore-producing fruiting bodies, similar to mushrooms. When present, these fruiting bodies can overwinter primarily on leaves, which then provide spore inoculum in the spring that can infect new leaves and flowers when conditions are wet and warm enough.
The stage at which these primary spores are being released from overwintering fruiting bodies is referred to as primary infection and causes lesions on leaves and fruits where new spores, known as conidia, begin to form and multiply. The process of primary spores being discharged can generally last until 1-2 weeks after petal fall. It can also be estimated using degree day (DD) accumulations starting at the green tip date, where most primary spores will be discharged by 400-600 DD, depending on weather conditions and other factors. The combination of the green tip date, tracking rain events, temperature, and degree days units, is how infection models like NEWA begin estimating percent primary spore, or ascospore, maturity and cumulative discharge throughout the season.
Infections that occur after this point are known as secondary infections and result from any conidia that established and multiplied based the original primary infections. Thus, early season management that eliminates primary spores from infecting apples is advantageous to keeping apple scab infections under control, especially in orchards with previous season infections. Which might be relevant this year. Last growing season, many parts of Minnesota experienced considerable amounts of rain events from spring until early summer, which impacted apple scab development in landscape settings and in orchards where the starting spore inoculum was high, or management practices were limited for whatever reason. Orchards in these circumstances may want to adhere to a more attentive management program this year.
Mowing or removing leaves of infected trees: If you think your orchard is at higher risk this year, collecting and flail mowing, or removing leaf litter is a cultural management practice that can help encourage leaf decomposition and reduce risk for apple scab infection occurrence. For mowing, it’s recommended to blow leaves into the aisle, or gather them by some other means.
Apple scab resistance: Certain apple varieties have some resistance to apple scab, which includes HoneycrispTM. Resistance is based on a single or multiple apple genes that help either stop or slow down apple scab disease development. One of UMN’s most recent apple releases, Triumph is considered more resistant to apple scab than other varieties like Honeycrisp, making it a potential option for growers with limited spray options. One way to increase apple scab resilience for new orchards, or new orchard plantings is to choose resistant apple varieties. This information can usually be found through nurseries suppliers and through fact sheets online.
Table from the Integrated Pest Management Manual for Minnesota Apple Orchards (2007).
Understanding the likelihood for apple scab infection events:
In the field of plant diseases, the concept of the disease triangle is essential to understanding pathogen risk development and likelihoods for infection. The disease triangle includes: 1) pathogens, 2) environment, and 3) host susceptibility. Pathogens are the disease-causing organisms, while the environment reflects the surrounding conditions that influence pathogen development, spore release, and the speed of infection. Finally, host susceptibility impacts how easily a pathogen can infect and continue to infect plant(s).When apple scab is present in an orchard, primary spore release and subsequent infections can happen after rain events and during wet conditions that have adequate humidity and warmth. Below is a table originally produced by Mills and La Plante that compares the amount of time it takes in hours for primary spores to cause an infection at a given temperature
Table 2. Apple scab likelihood for infection based on variations in temperature*
Table originally created by Mills and La Plante. An accessible version of this table may be found on the UC IPM webpage Apple Scab.
Recognize that this table can help growers estimate the risk or likelihood of primary and secondary apple scab infection, and help guide fungicide timing of applications. However, because it assumes continuous leaf wetness, the Mills-La Plante table may not fully reflect real-world conditions, which can lead to reduced accuracy in certain situations. For the purposes of this article, it can help to understand how increases in ambient temperatures can decrease the amount of time it takes for infection to occur during wet periods.
Chemical management:
For more information on chemical management for apple scab, visit the Midwest Fruit Pest Management Guide starting on page 14.To continue learning more about apple scab, visit our UMN Extension webpage, “Apple scab of apples and crabapples.”
Images: (Left) A recently-cut grapevine cane with a droplet of sap hanging from its underside, a sign the grapevine is exiting dormancy. (Right) A grapevine bud begins to swell in its development toward budbreak. Photos taken at the University of Minnesota Horticulture Research Center on 04/18/25 (UMN HRC; Chaska, MN, Zone 4b).
In southern regions of Minnesota, grapevine sap is beginning to flow, which is a sign that vines are exiting dormancy and grape buds are beginning to develop. As of last Friday, April 18th, many grapevine bud scales at the Horticulture Research Center were opening as the buds began to swell. At this growth stage, grapevine buds become more tender and vulnerable to rubbing off, which should be noted for producers who are doing any vine management at this time.
Grapevine growth stages noted in this newsletter series are based on the Eichorn-Lorenz system, often referred to as the “E-L Chart.” Similar to apples and other fruit crops, tracking grapevine growth stages is helpful to completing vineyard tasks, including early season management like shoot thinning, as well as pest management, plant nutrition sampling, and other tasks.
Resource: Eichorn-Lorenz (E-L) Grape Growth Stage Chart
Grapes
Growth stage update
Images: (Left) A recently-cut grapevine cane with a droplet of sap hanging from its underside, a sign the grapevine is exiting dormancy. (Right) A grapevine bud begins to swell in its development toward budbreak. Photos taken at the University of Minnesota Horticulture Research Center on 04/18/25 (UMN HRC; Chaska, MN, Zone 4b).
In southern regions of Minnesota, grapevine sap is beginning to flow, which is a sign that vines are exiting dormancy and grape buds are beginning to develop. As of last Friday, April 18th, many grapevine bud scales at the Horticulture Research Center were opening as the buds began to swell. At this growth stage, grapevine buds become more tender and vulnerable to rubbing off, which should be noted for producers who are doing any vine management at this time.
Grapevine growth stages noted in this newsletter series are based on the Eichorn-Lorenz system, often referred to as the “E-L Chart.” Similar to apples and other fruit crops, tracking grapevine growth stages is helpful to completing vineyard tasks, including early season management like shoot thinning, as well as pest management, plant nutrition sampling, and other tasks.
Resource: Eichorn-Lorenz (E-L) Grape Growth Stage Chart
Tying grapevine canes:
Are you in the process of tying down new canes to fruiting wires to establish new cordons, or to extend or replace already existing cordons? Don’t miss our recent article on UMN Fruit and Veg News, “Tips for securing grapevine canes this spring.”Resources for grape pest management:
Throughout the Fruit Update series, we’ll reference a number of different pests, including insect, disease-causing organisms, and competing plants to help you maximize knowledge and resources to support your pest management. This week we released an article with various resources to equip you with the knowledge and tools for grape pest management this season.Grape Flea Beetle: About and management options
Images: Grape flea beetles (Altica spp.) can lay their eggs on developing buds and bark, or on the underside of grapevine leaves as seen in the leftmost picture here. As adults emerge from winter, they feed on grape buds and mate. Damage to developing buds can interrupt shoot growth. Photos credited to T.L. Galvan and E.C. Burkness.
About:
Grape flea beetles (Altica spp.) get their name from their ability to jump similar to a flea when surprised. There exist two main species, which mainly vary based on where they lay their eggs (either on developing buds and bark, or on the underside of grape leaves). Their life cycles are similar in that they overwinter in various semi-sheltered environments as adults. These adults go on to feed on developing grapevine buds as they begin to swell in the spring, where buds are most vulnerable between the bud swell stage until the first leaf separates from the shoot tip. Damage can look like a small hole in a bud, but can interrupt shoot growth and lead to a reduction in yield. After feeding and mating, adult females lay eggs that eventually hatch into larvae, which can then feed on grape leaves, although the impact is minimal compared to bud damage in the spring.Management:
It’s important to monitor grapevine buds in the spring, especially along borders near wooded areas. When a threshold of 5% bud damage is noticed in a given region, management is recommended for that region (take note if any specific vineyard regions are being impacted each year). Conventional chemical management recommendations are provided in the Midwest Fruit Pest Management Guide on page 156, with its activity window found on page 154.Additional UMN newsletters to help you grow!
Our newsletter is one piece of the knowledge and resources it takes to successfully produce fruit. Below are additional newsletters offered by the University of Minnesota to support you.- UMN Small Farms Newsletter - A newsletter to stay informed about events related to food production.
- Minnesota WeatherTalk - WeatherTalk is a blog on climate and weather observations by retired University of Minnesota Extension climatologist and meteorologist Mark Seeley.
- Cottage Food Safety News - Be alerted to articles related to cottage foods production and safety.
Non-UMN Resources Cited for this Fruit Update:
Apple Disease - Apple Scab (Penn State Extension)
Apple Scab (University of California IPM)
Thank you to our farm and ag professional partners for contributions to the UMN Fruit Update series. Non-credited photos in this article were taken by Madeline Wimmer.
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