Editor’s note: This article originally appeared in the Winter, 2025 ONCore newsletter (Volume 29, Issue 1). We thought it was an excellent description of the integrated pest management process and where fungicide spraying fits in. It’s been modified from the original version.
Part One: Know Your Enemy
There is no denying that product efficacy and rotational partners are critical components of effective pest management. A pest is causing damage; we need as many tools as possible to control it. Let’s consider the basics of Integrated Pest Management (IPM). The first step in effective management is understanding the pest biology.
Let’s use apple scab as a case study for the IPM process. We’ll start with a deep dive into apple scab 101 by referring to its typical life cycle. Apple scab overwinters in infected leaves on the orchard floor. During the winter and early spring, immature ascospores (primary inoculum) are protected in specialized spore sacs, called pseudothecia.
Maturation of ascospores in the leaf litter on the orchard floor usually occurs at the same time the trees are emerging from dormancy. This means mature ascospores are present and ready to infect the first green tissue in spring. However, the percentage of mature ascospores in the orchard generally peaks when apples are at the late pink to petal fall stages of bud development.
Once the tree breaks dormancy and green tissue is present, a primary infection occurs if the following three conditions are met:
- Mature ascospores are present in leaf litter in the orchard.
- Weather conditions favour ascospore discharge and infection.
- Fungicide protection is inadequate to prevent infections.
Mature ascospores are discharged from the pseudothecia by rain and splashed up to emerging green tissue by wind. Moisture – dew or rain – is necessary for ascospore discharge and germination, as well as subsequent infection of apple tissue. Olive green, velvety lesions appear 10-28 days after infection by an ascospore, depending on temperature. The lesions initiated by ascospores result in primary infections, and in turn, produce spores called conidia.
Conidia are spread from primary lesions by rain or wind and initiate further infections when the combination of temperature and leaf wetness enables them to germinate and become established. These are called secondary infections, and generally occur within a tree or between adjacent trees rather than at a long distance.
The secondary cycle can be repeated many times during the growing season, whereas primary infection only continues until all overwintering spores are depleted. With frequent rainfall, the control of apple scab becomes extremely difficult as the season progresses, particularly if the disease becomes established from primary infections in the spring.
Early season management (green tip to tight cluster) provides the greatest economic protection against loss from scab control failure. In other words, don’t wait to get fungicide protection on!
Part Two: When To Strike
Just like how understanding the biology of the pest helps to determine appropriate intervention timings, understanding how fungicides work will help determine when best to apply them and ensure maximum efficacy (aka the most bang for your buck).
Fungicides can be divided into four categories, based on what they do:
- Preventative
- Curative
- Eradicant
- Antisporulant
Preventative
- Before the plant is even infected
- Before we can see any symptoms
- Most fungicides work preventatively
- If fungicides work in multiple ways, often they work best preventatively
Curative
- Stops the mycelial growth inside the plant
- Still can’t see any symptoms
- Fungicides with “kickback”
Eradicant
- Stops the pathogen during lesion formation
- Ok, now we can see symptoms
- Very few fungicides work this way, even though this is how we expect them to work
Antisporulant
- Stops the pathogen from sporulating
- We can see symptoms
- Several fungicides work this way, but your crop is already infected
In addition to the timing of a fungicide, efficacy can be affected by residues (or lack of), rains and risky gaps. A general rule of thumb often used is that 1 inch (2.5 cm) of rain removes approximately 50% of protectant fungicide residue and over 2 inches (5 cm) of rain will remove most of the residue. While systemic fungicides tend to perform better than protectant (or contact) fungicides in rainy periods, they do still require a certain amount of time prior to a rain event to be taken into the leaves – which isn’t always as easy as it sounds (see Part three).
During conditions conducive to disease development, it is important to maintain tight intervals between fungicide applications. Most labels will have the minimum interval listed. For protectant fungicides, a tight interval program would be 5-7 days. Where possible, do not extend intervals beyond 14-21 days if there are any concerns of disease. As a fungicide application ages, the efficacy of that product is reduced.
Spray coverage can be affected by wind in several ways:
- Wind direction – can carry droplets away from intended target
- Wind speed – affects how far the droplets travel
- Consistency – wind gusts can make coverage inconsistent
In addition to wind, spray coverage can also be affected by water volume, nozzle orientation, sprayer calibration, alternate row spraying, etc .
Areas of the canopy that are often missed due to poor coverage are within the tree due to spray not reaching through or at the top of larger trees . Routine monitoring can miss early signs of scab infection in these parts of the tree if not done thoroughly. I saw numerous situations this year (2025) where scab lesions were overlooked.
Part Three: Fungicide Playbook
Let’s take a closer look at some common scab fungicides and what is meant by contact and systemic activity and how they might redistribute after application. The rest of this article refers to pesticide brands available in Canada. They may have different names in other countries.
Contact (Protectant) Fungicides
These products remain on the surface of the plant tissue and provide preventative activity only. Examples of contact fungicides include fluazinam (Allegro, Vantana), coppers, sulphurs, mancozeb (Manzate, Penncozeb, Dithane), captan (Maestro, Supra Captan) and folpet (Folpan, Follow).
Unfortunately, because these products provide superficial coverage only, they can be prone to UV degradation or run-off and need frequent applications. Stickers/spreaders can help these stay on the plant but always refer to the label before using.
Systemic Fungicides
Systemic fungicides get taken up into the plant. Unlike contact fungicides, systemic fungicides tend to have longer duration and are rainfast once absorbed. However, sufficient tissue is needed for absorption so these products are best used after tight cluster in apples. How systemic fungicides move within the plant can vary:
Xylem-Mobile
Xylem-mobile, or acropetal fungicides move to the actively growing tips of expanding foliage and protect new growth. Examples of xylem-mobile fungicides include most Group 3 and 11s, as well as some Group 7s (e.g., fluopyram).
Translaminar
Translaminar fungicides move from the top of the leaf to the underside. Because of this limited movement, coverage matters. Examples of translaminar fungicides include most Group 7s as well as Cevya and Inspire Super.
Phloem-Mobile
Phloem-mobile, or “true” systemic fungicides move into the tissue and are carried to the roots to protect against root rots. There are no examples of phloem-mobile fungicides for apple scab. However, this group includes products such as Aliette and Phostrol which are registered for other diseases of apples. Unlike other systemic fungicides, this group has a short duration of activity (i.e., they move fast).
Resistance Management
Fungicides are grouped based on their mode of action, or how the product affects the disease. For example, all products in Group 3 have the same mode of action, so using one product is virtually the same as using all other products within that group. In pre-mix fungicides, both groups need to be considered in all rotation decisions.
One key strategy to good resistance management is rotating between products of different chemical groups. Figure 5 shows which fungicides belong to Groups 3, 7, 9 and 11. For instance, since Aprovia Top belongs to Group 3 and 7, it should not be followed by other Group 3 (Cevya, Fullback, Nova), Group 3+9 (Inspire Super), Group 7 (Excalia, Fontelis, Kenja, Sercadis), Group 7+9 (Luna Tranquility) or Group 7+11 (Merivon, Pristine).
For resistance management:
- Where possible, include at least half rate protectant fungicide.
- Do not use products containing the same chemical group in consecutive applications.
- Limit number of applications per group per season, where possible.
- Apply preventatively; do not rely on systemic fungicides for post-infection activity
- Do not use Group 3 (Nova, Fullback, Inspire Super) or Group 11 (Flint, Pristine, Merivon) fungicides after bloom for scab management as they are weak on fruit scab. Trials with Cevya have indicated good efficacy on fruit scab.
- Research from northeastern US indicates Group 7 fungicides may be weaker on fruit scab as well.
Part Four: Final Considerations
In conclusion, take some time to consider the following:
- Early intervention remains the cornerstone of effective disease management!
- Use weather monitoring tools to time fungicide applications
- Adjust spray schedules and product choice according to the weather
- Dedicate time for regular orchard inspections
- Train your team to identify symptoms early, accurately and consistently
- Optimize your spray program
- Protectant AND systemic fungicides
- Rotate classes to prevent resistance
- Select for broad-spectrum efficacy
- Reduce overwintering inoculum
The author gratefully acknowledges Katie Goldenhar, OMAFA Pathologist (Horticulture) for providing source material for this article.