Scab Lesions, Fire Blight Ooze, Captan Injury, Summer Rots

1. Scab lesions visible on ‘Macoun’ trees in Walden NY; Conida are available and can cause secondary scab infections

2. Fire blight ooze is available for secondary infections if first infections were established on 30 April – 2 May

3. Injury from captan – oil spray overlap or other mixes visible on leaves in some orchards

4. Summer rots and SBFS will require good cover spray schedules ahead


Figures 1 and 2 below of scab lesions, which are hard to find on some farms, were sent to us by Peter Jentsch. He found these on the lower canopy of old ‘Macoun’ trees in Walden, near the ground. He looked at the leaves on over a hundred of other trees on some farms and a number of different varieties throughout different orchards – with only these lesions found on these old trees. It seems most farms have done a great job covering against scab. Let’s keep it that way.

Figure 1. Chlorotic apple scab lesion in lower right part of ‘Macoun’ spur leaf image (Photo by Jentsch P. J., 2017)
Figure 2. Apple scab lesions on ‘Macoun’ spur leaf. Dark areas represent formation of asexual scab spores called conidia which will enable secondary infections (Photo by Jentsch P. J., 2017)


In our low-dose copper trial for evaluating management of fire blight, while we rated the disease incidence on flower clusters on 27 May, we found significant amount of fresh bacterial ooze on shoots and fruitlets (Figures 3 – 7). If fire blight infections were established on 30 April – 2 May,  ooze in unsprayed orchards has and will continue to allow further spread of bacteria in any windy and rainy conditions to establish secondary infections of soft shoots and rat-tail bloom. 

Figure 3. Shoot blight on ‘Honeycrsip’ with exuding white droplets of bacterial ooze on the top leaf petiole (Photo by Acimovic S. G., 2017)
Figure 4. Infected apple fruitlet of ‘Honeycrisp’ exuding white bacterial ooze droplets (Photo by Acimovic S. G., 2017)


Figure 5. Infected apple fruit cluster of ‘Honeycrisp’ exuding white bacterial ooze droplets (Photo by Acimovic S. G., 2017)
Figure 6. Rat-tail flowers on ‘Honeycrsip’ soon to be infected with fire blight from near by blighted cluster (Photo by Acimovic S. G., 2017)
Figure 7. Rat-tail flowers, here on ‘Honeycrsip’, are often visited by flies which can transfer fire blight ooze and thus infect them with fire blight bacterium Erwinia amylovora (Photo by Acimovic S. G., 2017)


In some locations, we have reports of signs indicating injury to leaves due to probably captan spray/s not separated far enough from oil sprays (Figure 8 and 9). It is also possible that some symptoms of injury are due to other complicated pesticide or adjuvant mixes or of cases of severe captan-oil injury.

Figure 8. Spots on ‘Red Jonaprince’ leaves indicating possible phytotoxic effect of captan and oil sprayed too close to each other, some other complicated pesticide mix or close-in-time overlap (Photo by Ellis N., 2017)
Figure 9. Dead edge patches on ‘Red Jonaprince’ leaves indicating possible phytotoxic effect of previously used complicated pesticide mix/es (Photo by Ellis N., 2017)


Further great read on this topic from Dr. David Rosenberger is available on:




Apple scab and other fungal diseases 

Between rains of 29 May to 1 June, we will accumulate between 0.25 – 1.5 inches of rain. It seems as a tough call for many of you to apply fungicide sprays on a tighter schedule than 7 days. Well, if you have done a good job so far on protecting your fruit from scab – don’t miss out on your sprays now: I stand behind my previous recommendation, that if your last scab spray was more than 7 days in past from 25 May, or manzate alone, you had to apply a spray of a kick-back fungicide with a contact as soon as leaves were dry on 27 May. Since the leaf canopy is larger now, it will hold a lot more fungicide residue and if you sprayed when dry on 27th May it should get you through the weaker infection period predicted for 29-31 May rains on most locations (some fungicide residue will re-distribute with rain). The next cover spray should not be stretched to 14 days after the last spray but done sooner, i.e. on 1-2 June when you will/should apply your codling moth insecticide as Peter Jentsch advised in his recent blog Codling Moth: Larval Emergence.
There are “many ways to skin a cat”, so fungicides and rates to use are your choice, as long as you follow the label limits. However, the remaining part of spring 2017 seems like a time where you would want to (1) use higher end of fungicide rates allowed by the label, especially if you had scab lat year (follow yearly rate limit), (2) keep up a good job on mixing contact with systemic fungicides, and (3) keep up a good job on alternating fungicide chemistries with different modes of action. You should take into account rust as still being a threat for infections 25-31 May, along with scab, and our general recommendation to you is: make sure you protect apple fruit well from both diseases and secure a good coverage while you spray your products. My main concern is that we had a lot of rain this spring with some hot periods. This means that apple scab and rust fungi had perfect conditions to produce and release copious amounts of spores to infect green tissue. Major scab infections happened 6-7 times so far, depending on a location. Granted, orchards that were clean from scab last year have somewhat lower chance to get scab infections now if you did a good job spraying early in the spring. However, I am not sure we want to take that bet now, in a year with so much rain, when apple scab fungus in leaf litter will reach its full genetic potential in ascospore production. Further on, scab lesions were visible from 10 May in lower Hudson Valley and if any sprays failed to provide good coverage and protection early on – conidia from these lesions are/will now be available to infect. If you looked at the powdery mildew model in RIMpro – we also had 4 major infection periods as well. On top of all that, black rot, blossom end rot, SBFS and other rots, had and will continue to have great chance to infect fruit. So keep up your sprays with appropriate fungicides, bearing in mind susceptibility of varieties you have to these diseases and rot history for your location. This is the year where there is no room to cut a fungicide spray here and there like 2016 was, or seemed to be. We will see what are we up against in the months to come.

Fire blight

If any flowers are/were still open, and if streptomycin was not sprayed – infections could have taken place 25-28 May. If flowers are still opened or yet to open in newly planted orchards – weather predictions indicate possibility of infections for 30 May – 1 June rains/wetting.

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