Powdery mildew overwinters as fungal strands (mycelium) in dormant buds that were colonized the previous summer. When the buds open in spring, the fungus grows on the new plant tissue producing chains of conidia, giving the leaves a white powdery appearance (Fig 1). Only new leaves are susceptible to infection and only for a few days after emergence – but fruit become infected between pink and petal fall. The pathogen continues to infect as long as shoot growth continues.
Conidia produced by overwintering mycelium serve as the primary inoculum in spring and are easily dispersed by wind currents to susceptible tissue. Being wind dispersed, powdery mildew is considered a “neighbourhood issue” as it can move many kilometers on air currents to nearby orchards.
Conidia landing on susceptible tissue germinate when temperatures are between 10-25ºC and high relative humidity (optimal 90%). However, some conidia germinate when relative humidity is as low as 70%. Unlike other disease-causing fungi, leaf wetting is not necessary for powdery mildew infections. In fact, conidia will not germinate in free-standing water. Rain or dew frequently washes the powdery mildew conidia off leaf surfaces.
Colonized shoots and buds – particularly on young trees – have reduced vigour, are less productive and more susceptible to cold injury. Temperatures below -24ºC kill most infected buds. Although cold winters reduce the survival of infected bud, they also reduce the inoculum potential of the disease.
Why Was Powdery Mildew So Bad in 2021?
- Mild winter – With the mild 2020/2021 winter, there was greater survival of the overwintering mildew infected buds contributing to higher primary inoculum. This meant that when buds broke dormancy early spring, the fungus was able to quickly resume growth and colonize developing leaf tissue. If protectant fungicide coverage wasn’t adequate, the infection was able to take hold.
- Dry conditions – Often called the “dry weather disease”, powdery mildew does not require moisture for germination to occur. The moderate spring temperatures with little rain but high humidity – especially at night – made for ideal infection conditions.
- Relaxed fungicide intervals – With the moderate and dry spring conditions in 2021, risk of apple scab infection was relatively low. Like most diseases, scab requires leaf wetness to germinate. Unfortunately, when scab stays away, mildew comes to play. Without the risk of scab infection this spring, fungicide application intervals did not always stay consistent or frequent enough to keep new growth protected during optimal conditions for mildew infection.
Timing Take Home
Key timings to remember for effective powdery mildew control:
- Be prepared early – Infection can happen as soon as buds begin to break if overwintering inoculum is present, especially on susceptible cultivars. Control can be harder to manage later in the season if early sprays are missed.
- Wet weather = scab BUT dry weather = powdery mildew – Maintain tight intervals (7-10 days) with effective mildew products during critical growth stages (ie., until terminal set) even when scab infection risk is low.
- If shoots are growing, infection is possible – Mildew requires actively growing tissue. Shoots are at risk of infection until terminal bud set. If growth resumes later in the season, infection can occur at that point as well.
For more information on effective mildew fungicides and susceptibility of common apple cultivars, see Table 3-5. Activity of Fungicides on Apples Diseases and Table 3-8. Disease Susceptibility Ratings of Common Apple Cultivars in the 2021 Publication 360A, Crop Protection Guide for Apples.
Stay tuned for the February 2022 issue of Orchard Network Newsletter where the spotlight will be on key timings for fire blight and summer diseases.