PROS AND CONS OF PROLINE

Patty McManus, UW-Madison

June 2019

Proline (prothioconazole) fungicide was introduced in cranberry production a few years ago. In many trials it has been the top performer for fruit rot control and it has been as good as the industry standards (Orbit/Tilt and Indar) for cottonball control. Thos are the “pros” of Proline.

         The “cons” are that work by Christell Guedot’s group has shown a significant decrease in the amount of cranberry pollen collected by honey bees 24 and 48 hours after application of Proline (5 fl oz/acre). They did not collect pollen samples later than 48 hours because growers needed to use other products that would have made it impossible to compare beds at later time points. Thus, just how long the negative effect of Proline persists is not known. For more information on that work, see 2019 Cranberry School Proceedings, pages 2-3. Although that study showed Proline deterring honey bees, we have included Proline in several fungicide trials since 2011 and have never found it to reduce yield. How do we reconcile what seems to be contradictory results?  Here are a few possible explanations:

·      McManus et al. fungicide trials were done by treating small plots, with the entire experiment taking up a small fraction of a cranberry bed. By contrast, Guedot et al. collected data at marshes where Proline was applied to whole beds. Perhaps the amount of Proline applied to several small plots intermixed with plots treated with other fungicides was not enough to deter bees. But when whole beds are sprayed, the amount is great enough to deter honey bees.

·      Guedot et al. measured amount of cranberry pollen collected, but they did not measure yield. Although the amount of pollen collected declined after Proline was applied, perhaps the deterrence, although statistically significant, is not sever enough to inhibit yield. They did not measure yield because cooperating growers needed to apply other peseticides later in the season, which would have made it impossible to sort out the effect of Proline on yield.

·      Yield in the McManus et al. fungicide trials was determined by weighing berries hand harvested from very small plots = 1 or 2 square foot areas. There sometimes is great variability in yield even among replicate plots of the same treatment. We do not know how well our yield estimates based on such small sample areas “scale up” to whole beds. It’s the best we can do, but it’s far from perfect.

·      Perhaps pollinators other than honeybees are not deterred by Proline, and they are doing enough pollination so that yields were not reduced in the McManus et al. fungicide trials.

·      One might speculate that the amount of cranberry pollen collected by honeybees drops off after 24 or 48 hours, simipley because there is less pollen available as bloom advances. This is likely NOT the case, however, because dGuedot et al. also monitored pollen colletion after treatment with Abount + Indar and did not see a significant reduction in pollen collection at 24 and 48 hours after treatment with that funtgicide combination.

Aware of possible concerns with fungicides and pollinators, in 2017, Jack Perry and I tested the efficacy of fungicide programs that varied in when two sprays were made: twice before bloom; twice during bloom; or twice after bloom. We rated fruit rot control at three locations and cottonball control at one location. In 2018 we tested those three spray timings and a fourth in which fungicides were applied just before bloom and again during early bloom. We added a second cottonball site in 2018 as well.

         In general, the best level of fruit rot control with Proline (and other fungicides) was achieved when sprays were applied during bloom rather than before or after, but the difference was often not statistically significant. Sub-optimal timings (ie., before or oafter bloom) did consistently control fruit rot significantly better than the untreated check. Similarly, cottonball control was best when fungicides were applied during bloom, which was expected since the cottonball fungus mimics pollen by germinating on flor stigmata and growing down the style to developing ovaries. For cottonball, spraying before bloom was usually as effective as spraying during bloom, but spraying only after bloom resulted in cottonball similar to the untreated check in several cases. The data tables are quite “busy”, so I don’t include them here, but please contact me (pmcmanus@wisc.edu) if you want to see the detailed reports that include not just Proline but some other fungicides as well.

         Further work to determine the relative risks and benefits of fungicide use is warranted. But in the meantime, what should you do? I believe that if you have disease bad enough to justify fungicide use, then you should apply the fungicide at a timing that will provide the best level of control. For cottonball, that is during bloom, and for fruit rot, at least one applications should be during bloom. If you want to avoid bee deterrence by Proline, you could apply a different fungicide during bloom and follow up with Proline at late bloom or after bees are gone during early fruit set. The Guedot work showed that Abount + Indar did not deter honey bees. However, with that combination or Quadris Top, there is a 14-day water-holding restriction. As mentioned above, applying yield information from small plots to whole beds is imperfect. In trials conducted over many years, we sometimes have rot or cottonball around 30% with no statically significant yield loss, but I would never suggest that 30% disease is acceptable! Howver, when we do detect a significant yield loss among fungicide treatments, it has almost always been because of high disease in the untreated controls or an ineffective fungicide treatment. We have never seen a loss in yield with Proline or other effective fungicides.