Apple Disease Management: Part 2 – Tight Cluster to Petal Fall

Tight cluster to petal fall can get quite complicated for disease management with 1) rapid growth occurring during a critical infection period, 2) more diseases to deal with (scab, mildew, fire blight, rust), 3) resistance management to consider, 4) keeping fungicide groups straight (especially pre-mixes) for proper rotation, and 5) certain tank-mix incompatibilities now that more orchard activities are underway including thinning.

The following are some disease management considerations during this time of very rapid growth. Refer to Table 3–14. Activity of Fungicides on Apple Diseases in Publication 360, Guide to Fruit Production to assist with choosing the best product for the most likely diseases. Consider the history of the disease, resistance management strategies, as well as the activity of each product to diseases and the weather.

Scab and Powdery Mildew

 By tight cluster, apples are typically entering into a time of critical infection period for scab and powdery mildew with higher daily temperatures, large amounts of lush growth and rapid maturation of spores. Management programs at this point should begin incorporating systemic fungicides (Groups 3, 7 and 11). However, unlike the protectant captan and EBDC fungicides, which have multi-site activity and low resistance potential, systemic fungicides are typically single-site and are at high risk of resistance development. For resistance management, half to full rate protectant fungicides should still be included with all applications.

While captan does not provide effective rust control, EBDCs do. Be sure to keep EBDCs as the protectant tank mix particularly at petal fall if rust is a concern in your orchard. It’s best to avoid the use of captan in general during this growth period to avoid phytotoxicity by complicated tank-mixes, use of adjuvants or other incompatibility issues. Instead, save captan for summer disease control.

Succinate Dehydrogenase Inhibitor (SDHI) fungicides

 The SDHI fungicides (Group 7) are the newest group of systemic fungicides including Fontelis, Aprovia, Sercadis, Kenja 400SC, Luna Tranquility (Group 7+9) and Pristine WG (Group 7+11). These products are registered for control of apple scab, powdery mildew and rust (Fontelis only). Some formulations within this group contain mineral oil or require a surfactant, which can result in injury under certain conditions if used in combination with captan.

At the present time, there is no known resistance to any Group 7 product in Ontario. For this reason, more applications of these products can be used, compared to the Group 3 and 11 fungicides. A maximum of 4 applications of Group 7 fungicides is allowed each season, with ideal placement between tight cluster and first cover. However, particularly with Luna Tranquility (pre-mix containing Scala), it is best to use these fungicides before petal fall as they have only fair to moderate activity on rust, if this is a pest of concern. Often rust is most difficult to control on leaves after petal fall because of rapid shoot growth.

Pristine WG is also a very effective product for the control of fruit rots. Since this pre-mix product includes Group 7, it may be worthwhile to save 1-2 applications for summer use.

As with all sound resistance management practice, do not apply consecutive sprays from the same fungicide group. This is where Group 3 and 11 fungicides can come into play with product rotation.

Sterol Inhibitor (SI) fungicides

The SI fungicides (Group 3), now including Nova, Fullback 125SC and Inspire Super (Group 3+9) have been a standard for control of apple scab, powdery mildew and rust for many years. Resistance has been documented in Ontario scab populations and may also be present to a lesser extent in powdery mildew populations, though further testing is needed. For this reason, SIs should not be used in orchards with resistant scab populations and should always be tank-mixed with a protectant fungicide. The benefit to keeping Group 3 and 11 fungicides in rotation is the good powdery mildew control they offer (so long as resistance is not present) as well as to help reduce resistance pressure for the SDHI fungicides.

Since SIs are not effective on fruit scab, products from this group should not be used after bloom.

Strobilurin fungicides

Similar to the SIs, the strobilurin fungicides (Group 11) such as Flint, Sovran and Pristine WG (Group 7+11) have been used for many years for control of apple scab, powdery mildew and rust. Scab resistance has also been documented in Ontario to this group, so tank-mixing with a protectant fungicide is critical. Where strobilurins are working, their best fit may be at petal fall and/or first cover to avoid using them when scab pressure is of greatest concern.

As mentioned earlier, Pristine WG is registered for control of summer fruit rot. For more information on this timing, see Apple Disease Management: Part 3 – Summer to Harvest.

Resistance management

One key strategy to good resistance management is rotating between products of different chemical groups. This does not just mean rotating from one product to another. Fungicides are grouped based on their mode of action, or how the product actually affects the disease. For example, all products in Group 3 have the same mode of action, so using one product is virtually the same as using all other products within that group. In pre-mix fungicides, both groups need to be considered in all rotation decisions. Figure 1 shows which fungicides belong to Groups 3, 7, 9 and 11.

Figure 2 depicts a summary program that would have activity on scab, powdery mildew and rust while providing excellent resistance management so long as there is rotation among the higher risk fungicide groups (ie., SI, SDHI, strobilurin).

Fire Blight

While the fire blight bacteria are always present in an orchard, the risk of infection varies year to year depending on environmental factors, management practices and overall tree health. Unfortunately, many areas of the province had ideal conditions at bloom last year for fire blight infection, including a long secondary bloom. Open blossoms are the most susceptible tissue to fire blight infection since they provide an opening for bacterial entry.

The first line of defense for fire blight management in an orchard is prevention. There is no silver bullet to eradicate this disease once it becomes well established. Protect blossoms with timely applications of registered products such as a rotation of antibiotics (Streptomycin, Kasumin), copper (Cueva) and/or biologicals (Blossom Protect, Double Nickel, Serenade OPTI).

The following are some key points to consider for effective fire blight control during tight cluster to petal fall:

• Closed blossoms or flower cup after petals have dropped are not susceptible to infection.
• Use a forecasting model such as CougarBlight, Maryblyt or the OMAFRA Fire Blight Prediction Maps.
  • If you haven’t used one of these programs before, spend some time prior to bloom getting comfortable using the model so you are ready to go for infection time.
  • Prebloom information may be required depending on the model you are using.
• Ensure adequate water volumes are used and that sprays are deposited into blossoms. Do not use excessive air speed on sprayers as this will propel sprays past the flower targets. For more information on calibrating an airblast sprayer or troubleshooting coverage, refer to Airblast 101 Handbook .
• Products are most effective when applied just prior to a wetting event such as dew or rain.
  • Since biologicals have a preventative action, these products should be applied when the forecasting model says risk is coming in the next 3-4 days or between 10-50% bloom.
  • Placement of biologicals at early bloom will provide protection during lower infection periods and allow use of antibiotics to be saved for later bloom when infection risk tends to be higher.
  • Antibiotics will provide activity 24 hours before and after a wetting event.
• Products should be re-applied every 2-3 days during times of infection risk due to product breakdown and presence of new blossoms.
• Copper and some fungicides are not compatible with biologicals. Check the label.
• Secondary blossoms should be removed immediately since they rarely produce marketable fruit and are more of a liability. Otherwise, continue with a bloom control program when a forecasting model predicts fire blight infection risk.
• Consider managing growth of succulent shoots by limiting excess nitrogen. Apply only when warranted by annual leaf analysis.
• Incorporate Apogee beginning at king bloom petal fall to reduce the vegetative growth of susceptible shoots and stimulate the plant defense system to reduce infections.