Remember this? This is what apple scab can do to many of our older, susceptible apple, crabapple, hawthorn cultivars after cool, wet springs. Disease inoculum (sporulating structures called “pseudothecia”) can be found inside overwintered fallen leaves from 2020, waiting for enough heat and moisture to be able to be fertilized, mature and release infective two-celled “ascospores” in the rain and wind. (If you inspect old apple leaves, the pseudothecia resemble black dots.) If we get another cool spring with frequent precipitation during leaf emergence periods, we could be heading into another perfect storm for disease.
For MORE information on fruit tree production and pest management, Check out the ONFruit blog post on apple diseases
Apple Scab (Venturia inaequalis, on crabapple/apple/hawthorn/mountain ash, see photo above) and Fireblight (Erwinia amylovora, on apple & pear, see photos below) diseases will being sporulating and infecting green tissue and flowers starting at bud swell and green tip. Fireblight is most infectious starting around the beginning bloom (including re-bloom) of apple and pear, but the infection period can last weeks and be facilitated by pruning or other wounding events, such as wind and hail damage.
Bacterial Fireblight requires warm, moist conditions to successfully infect host tissue. Temperatures need to be greater than 18C for fireblight bacteria to be active and the risk of fireblight will increase with increasing temperatures, peaking at 24C. Leaf wetness periods during bloom will also increase the success of those primary floral infections of fireblight bacteria.
Management programs are different for each disease. This is because:
- Apple scab is caused by a fungus and Fireblight is caused by a bacteria, and
- For Apple Scab, the disease enters through emerging leaf tissue (and fruit tissue) and for Fireblight, the disease enters through the floral stigmas
APPLE SCAB on Apple, Crabapple, Hawthorn (photo above). Protect susceptible new leaf tissue starting at Green Leaf Tip, especially before precipitation events. Spore release peaks during bloom, making it the most important timing for fungicide applications. Repeat fungicide applications every 7 days until petal fall (leaf expansion). Remember that 2.5 cm of rainfall is enough to wash away protectant fungicides. Apple Scab products include Nova, Banner (grp 3), Pristine (grp 7,11), Compass (grp 11) and Captan (grp M4). Some newer fungicide options include Luna Tranquility (grp 7,9) and Aprovia Top and Inspire Super (grp 3, 7). The biological Serenade (grp 44) is an option where traditional fungicides are not desirable. During cool, wet leaf emergence periods, 3 fungicide applications can really help reduce disease incidence. Always rotate fungicides between chemical groups/families. Once leaves harden off, they are much less susceptible to apple scab.
FIREBLIGHT. Floral applications of products for Fireblight (such as copper, Kasumin, streptomycin) should begin at early bloom and continue throughout the bloom period to reduce numbers of Fireblight bacteria on floral structures (stigmas). The floral stigmas are the main entry point for Fireblight into the tree. Secondary summer infections may be possible in June/July where storm damage (e.g. hail, wind) has occurred, creating wounds for entry of Fireblight bacteria into new branches.
And don’t forget those Gymnosporangium rusts:
Pear Trellis Rust on Pear in Summer (photo above). When pear leaves start to emerge from buds and precipitation is expected, protect new foliage against Pear Trellis Rust (Gymnosporangium fuscum) infections. These spores can travel long distances through weather currents. Pear trellis rust products include Nova (grp 3) and Pristine (grp 7,11). Protect new foliage with 2-3 applications, focusing applications just before a precipitation event.
Cedar-Apple Rust, Cedar-Hawthorn Rust & Cedar-Quince Rust in Summer (above). Where leaves are starting to emerge on crabapple, hawthorn and serviceberry (Rosaceous hosts) and precipitation is expected, protect new foliage against Gymnosporangium rusts. Gymnosporangium rust products include Nova (grp 3), Pristine (grp 7,11), Daconil (grp M5). Protect new foliage with 2-3 applications, focusing applications just before an precipitation event.