Leaving behind the fourth or fifth wettest April, rains continue in May. Many growers are confronted with serious issues with respect to scab management. Depending where you are in the state, development is at anywhere from petal fall to 2nd cover (kings at 10-11mm) or even further along. Against some pretty grim probabilities, most of the state managed to not suffer a freeze and hopefully everyone was able to protect their crop.
If our weather remains cool and wet, any scab that became established during April will continue to spread during summer, increasing the potential for late-season fruit infections. What is a grower to do? As we all know, apple scab is best managed preventatively. Unfortunately, the weather has prevented this best management practice. In the northern part of the state, which is anywhere from tight cluster to first cover, Inspire Super at the highest labeled rate with mancozeb or captan for two sprays will protect against scab, along with rust (Inspire is less effective on mildew, although with all this rain mildew is unlikely to be a problem). Inspire Super contains Vanguard as a tank-mix partner, taking the fungicide resistance pressure off SI component (difenoconazole) for any infection events that occurred within 72 hr prior to the application. For growers searching for a different option, as long as temperatures remain below 70, Scala or Vangard alone can provide up to 72 hr of reach-back activity if applied before bloom, but Scala and Vangard have almost no value for stopping scab if infections have more than a 72-hr head start and they do not redistribute well to protect new growth. A final option, for growers with no history of dodine resistance, is Syllit. Keep in mind that these fungicides (Scala, Vanguard, Syllit) should be used in combinations with mancozeb or captan both for resistance management and to prevent total disaster if the block has more dodine-resistant scab than expected. This will be an expensive spray, but well worth the investment. Any breakthrough scab should be deactivated, thereby preventing secondary infections from occurring. When using any fungicide as a rescue treatment, always used the highest labeled rate allowable.
In the southern part of the state, which is at 1st to 3rd cover, the most critical time of scab control has actually passed (apple scab generally remains a threat for at least three weeks after petal fall). The ascospores may not be shooting, but with all the rains the previous 2 weeks, and skepticism that we would even have a crop, it is unlikely that anyone approached 100 percent control of primary scab, under perfect scab-infection conditions. Any remaining ascospores more than likely managed to infect those rapidly growing terminal shoots and young, succulent leaves. Orchards that are carefully scouted three weeks after petal fall and do not show scab are unlikely to develop severe scab problems later in the season—Except, our cool wet spring and almost 2 weeks of heavy rain has probably added at least 7-10 more days to that three week cut-off. Even though many of you are three weeks past petal fall, keep on the look out for scab. Remember that after petal fall, captan used at full rates will be the most effective way to keep scab off of fruit if any infections become established on leaves. As a bonus, it’s great against bitter rot, too!
Keep in mind that that the scab risk doesn’t disappears until we get some hot, dry weather. Secondary scab conidia production drops off rapidly after several days of mid-80 degree temperatures. Furthermore, conidia viability is reduced with increasing temperatures, minimizing the risk of infection as well.
Rescue: When to implement a rescue treatment
For the purposes herein, rescue treatments are those applications of fungicides that are needed after an excessive rain event, or in the case of intermittent rains early in the season that promote scab infection, but not enough rainfall for redistribution of mancozeb or captan to prevent infection. Normally, we recommend that fungicides for rescue treatment should never be applied if excessive scab is present in the field, nor should the eradicant “kick back” fungicides (SI’s, strobilurins, dodine) should be applied once sporulating lesions are found in the orchard as this will rapidly promote the development of resistance.
However, doing nothing isn’t a very good option. Furthermore, these applications of rescue treatment fungicides will protect newer infections from developing, although you may be driving resistance by applying fungicides to already sporulating lesions. This will reduce the incidence of lesions this year, but possibly select for fungicide resistance in those late, already sporulating lesions. For those in the southern part of the state, keep in mind that the newer SI fungicides do not work as well against fruit infections compared to leaf infections.
Although strobilurin resistance is not widespread in Indiana, the use of strobilurins would not be recommended as they have even more limited “kick back” activity than the DMIs. This leaves many growers with two choices: Inspire Super or Syllit.
Although exact numbers are unknown, the number of rescue treatments a grower has is finite, with between four to six rescue treatments estimated with the newer SI fungicides (difenoconazole, Inspire Super and fenbuconazole, Indar) before even they will no longer work in an orchard with fungicide-resistant scab. The newer, ‘Second Generation’ of SI’s are much more active against scab, and have been shown to control scab that is resistant to Rally/Nova and Vintage/Rubigan in some (but not all) experimental orchards with high population of SI-resistant scab. In those orchards without a history of dodine (Syllit) resistance, dodine can be used to “burn out” existing lesions. Our research in Indiana has found that approximately 35% of the isolates tested possessed varying degrees of dodine resistance, from reduced efficacy to full resistance, but that resistance to dodine was much lower than resistance to Rally. Finally, Yoder et al. (2009) found Syllit, Indar and Pristine all significantly reduced scab incidence when first applied three weeks after the first scab lesions were observed, however the level of fungicide resistance in this orchard was expected to be minimal at the time of application.
When applying a rescue treatment, fungicides should be applied as soon as entry into the orchard is possible, and best results will occur if fungicides are applied no later than 72 hours after the start of an infection period. For Inspire Super and Syllit, the sooner the spray is applied after the infection period, the more effective it will be. Fungicides applied in a post-infection program should be used at their highest recommended rates. Syllit (Dodine) has excellent eradicant properties, if applied early in the season, and anecdotal evidence from growers suggests it is very effective later as well. Growers who are uncertain as to their orchard’s resistance status should use it in combinations with mancozeb or captan. Newer formulations of Syllit should minimize the risk of russetting on Goldens, and our own work last year did not identify any problems using captan and Syllit together. Apply at highest labeled rates to eradicate any early infections and combine with mancozeb or any strobilurin for resistance management. Post-infection sprays should be followed with a second application of the Syllit or Inspire Super + protectant (mancozeb or captan) approximately seven days later to make sure that scab lesions are completely killed (this assumes that dodine or SI resistance is not an issue).
It is important to note here that the curative or “kick-back” ability of strobilurins was never as effective as the SI fungicides. Thus, using Inspire Super (which field studies have regularly shown to have the best efficacy against apple scab), or Syllit for scab control is what I believe is the better option when one wishes to maximize activity against apple scab infections that might already be present in leaves.
One obvious problem with this spray program is that extended or severe rainfall can interfere with post-infection applications that must be made within 72 hours after the start of an infection period. Unfortunately, extended periods of rain are a problem with any fungicide program, as we have all just witnessed.
A final note: This is my last article for Facts for Fancy Fruit, as I will be retiring from Purdue May 12. I want to thank everyone for sharing their fruit growing path with me—I learned so much from all of you, and I hope you have learned something from me in return. I am so very grateful that you let me into your orchards, vineyards, berry patches and lives, and shared with me the goodness of flowers, fruit and hope. So, thank you, and I hope we meet again!