Apple Disease Management: Part 3 – Summer to Harvest

The following is a continuation in the Apple Disease Management series. Refer to earlier posts Part 1 – Early Season (Green Tip to Tight Cluster) and Part 2 – Tight Cluster to Petal Fall for more information.

Apple Scab

With the significant amount of rainfall this spring, many areas have now reached over 50% ascospore maturity. The end of primary scab season can be determined through the use of the ascospore maturity degree-day model. For more information, refer to Notes on Apple Disease: Apple Scab in Publication 360, Guide to Fruit Production. At 418 DDC, over 95% of the ascospore supply should be depleted if sufficient rain has occurred. After this point, wait 2 weeks and then check the trees for scab. It can take 10-28 days for lesions to appear after a scab infection period depending on the temperature. Be sure to check the top of standard trees or anywhere else spray coverage may not have been adequate, such as a thick tree canopy. Primary scab is often overlooked in these areas and can come as a surprise when lesions suddenly show up on the fruit late season.

While scab infection risk is lower in cooler weather, it is still possible if a wetting event is long enough to meet the requirements for infection. Many regions experienced this in recent weeks with wetting events lasting for more than 48 hours. The Revised Mills Table outlines the hours of leaf wetness required for scab infection to occur as well as when lesions would likely appear.

If primary scab was controlled, the rates of fungicides may be reduced and the interval between sprays may be lengthened for the remainder of the growing season. Following petal fall or first cover, EBDCs are no longer an option due to their 45-day preharvest interval. Summer applications of captan, however, will provide efficacy on scab, sooty blotch (Figure 1), fly speck (Figure 2), black rot (Figure 3) and bitter rot (Figure 4) depending whether the full rate is used. If summer oil or other tank-mixes that are incompatible with captan are used, rotating with Granuflo-T (28-day PHI), Allegro 500F (28-day PHI) or Pristine WG (5-day PHI) will also provide summer disease control. Allegro 500F will provide suppression of European red mite as well.

Powdery Mildew

Powdery mildew does not invade mature leaf tissue, so spread ceases when trees stop producing new terminal leaves. Continue to include a fungicide with mildew activity such as Pristine WG until terminal bud set. Summer applications of Purespray Green Spray Oil for mites and aphids will also have suppression activity on mildew.

Fire Blight

If shoot blight is present in the orchard, appropriate summer management decisions are critical to prevent further spread of infection. Michael Celetti, OMAFRA’s plant pathologist posted some helpful tips for removing fire blight strikes in 2015. Some key points include:

  • Prune out infections early, but only if there are just a few strikes per tree.
  • Excessive pruning can stimulate growth, potentially making the situation worse.
  • Cut at least 30 cm beyond the water soaked margin of the infected shoot.
  • Only prune out strikes when there are 2-3 consecutive days of low humidity and temperatures below 25⁰C.
  • Leave cuttings in the row middles to dry up before removing from orchard.

As mentioned in Apple Disease Management: Part 2 – Tight Cluster to Petal Fall, Apogee can be used to reduce vegetative growth susceptible to fire blight infection. Applications should begin at king bloom petal fall and continue every 14-21 days as needed.

The use of Cueva and/or Double Nickel will also help limit the spread of bacteria to healthy shoots. Keep new pruning cuts covered if conditions for infection persist. While the risk of russeting with Cueva is lower than other copper sprays, there is still the possibility of damage particularly if applied during poor drying conditions or at a high rate. However, fire blight management at that time of the year comes down to saving the tree rather than keeping quality of fruit.

Summer Diseases

While infection from fly speck and sooty blotch generally occur around petal fall, these are often considered summer diseases since symptoms appear on fruit midsummer to harvest. Fungicides applied during primary scab infection period will have efficacy on fly speck and sooty blotch. However, once scab management relaxes, fungicide sprays should be initiated in late summer and continue through to harvest.

Black rot and bitter rot applications should be made every 10-14 days during warm summer months but shortened to 7 days if frequent rain is experienced. If possible, timing an effective fungicide application just prior to a rain event or thunderstorm will protect fruit from rain-splashed spores. Trials in Ontario have found Allegro 500F and Pristine WG to be very effective against bitter rot, while Granuflo-T provides suppression only.

A significant part of fruit rot management is through good orchard sanitation. Removing old cankers, as well as dead limbs or trees will reduce the potential primary source of disease in the orchard. Mulching or removing infected fruit on the orchard floor will also help reduce inoculum and the potential of spreading the disease. This includes any fruit removed in-season while hand thinning.

During times of extreme heat, sun protectant products may help boost the immunity of the fruit and protect against bitter rot infection. Bitter rot lesions are often more common on the sun-facing side of ripening fruit. Research has shown that fruit surface temperature can be 14-17°C higher than air temperature. This, in turn, has been found to make fruit more susceptible to bitter rot infection. Having a clay- or calcium carbonate-based sun protectant product applied prior to a heat event will reduce fruit surface temperatures.

With the number of activities going on in the orchard late summer, products with short re-entry and preharvest times are often required. As well, there may just be a need for a suppression product to fill in the gaps between intervals or before harvest. In these cases, biopesticides may be a good alternative. For more information on products available for scab and summer diseases, see the next (and final) post of this series, Apple Disease Management: Part 4 – Biofungicides.

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