Remember this? This is what apple scab did to many of our older, susceptible crabapple cultivars in 2017. Disease inoculum can be found on overwintered foliage, waiting. If we get another cool spring with frequent precipitation during leaf emergence periods, we could be heading into another perfect storm for disease.
The way I see it there is only one way to be…..PREPARED.
Apple Scab (Venturia inaequalis, on crabapple, see above) and Fireblight (Erwinia amylovora, on apple & pear, see below) diseases will being sporulating and spreading starting at bud swell and beginning bloom of apple and pear.
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. Protect new leaf tissue starting at Green Leaf Tip, especially before precipitation events. Repeat fungicide applications every 7-10 days until petal fall (leaf expansion). Apple Scab products include Nova, Banner (grp 3), Pristine (grp 7,11), Compass (grp 11) and Captan (grp M4). Some newer fungicie options include Luna Tranquility (grp 7,9) and Fontelis (grp 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.
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) 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 (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.