We are starting to see a sudden surge of bacterial spot on leaves and fruit of peach. I visited an untreated O’Henry block yesterday in Fort Valley, and virtually all the fruit and most of the leaves were covered in bacterial spot. The images below show spotting of fruit and leaves as we observed them yesterday. The Augustprince block, immediately adjacent to this unsprayed O’Henry block, was in good shape, with virtually no bacterial spot and limited copper injury on leaves. The producer indicated that the O’Henry had no bacterial spot until the recent rains started. I believe this is additional evidence that Xanthomonas will infect peach fruit and leaves late in the season. As we are now in the final push for many of our cultivars, what should we do to address the risk of bacterial spot during the 21-day preharvest period? That was the question I was asked, and it does not have a perfectly clear answer. I will tell you what I know, what I don’t know, and what I speculate.
Though we have often ignored this late timeframe for bacterial spot development, I don’t think you can rule it out completely. One can’t use oxytetracycline products and some copper products, due to a 21-day preharvest interval (PHI) restriction. With that said, some copper products (e.g. Badge SC) have a 0-day PHI. However, California recommendations for avoiding “inking” say specifically, “Do not spray foliar nutrients or preharvest fungicides containing iron, copper, or aluminum within 21 days of predicted harvest,” so what should we do? I think the evidence that copper specifically causes inking is somewhat weaker than that for other heavy metals. There is perhaps a stronger association for inking when heavy metals are combined with abrasion, as opposed to heavy metals alone. Without regard, it is a judgment call as to whether one should apply copper within the 21-day window.
The fact of the matter is that we do not have good, research-based information on the application of copper in preharvest sprays. I hate for us to trade one devil for another, but with the weather conditions we are currently observing, wet with inundating rains, additional applications of copper could possibly help us to avoid either minor or major downgrades of fruit due to bacterial spot during the next month or two. In discussions with other specialists and researchers, we generally think that bacterial spot infection can persist within the 21-day period prior to harvest, and that additional applications of copper could help suppress it during the preharvest period. The time from infection to symptom development on fruit is somewhat unclear, but warm/hot conditions will decrease this timeframe. Estimates range from 3-7 days to two weeks. That means infections that occur in the last week prior to harvest are of limited or no consequence; those that occur at 21-14 days prior to harvest might be visible at harvest if field conditions are hot and wet. Therefore, if you are considering applying copper in the last 21 days, you could apply one application at 21 days prior to harvest (oxytetracycline can also be applied at 21 days prior to harvest, so one or the other will suffice) and another at 14 days prior to harvest; that would likely do it. I think the evidence that copper causes damage to fruit is low, and we are always applying very low rates of copper at this time of year, but I am not willing to guarantee that inking would not occur. I doubt it, but I don’t know for sure. I also have a gut feeling that using copper could be valuable this year if the rains continue. As I said, it is a judgment call. If we suddenly see drier conditions, we could hold off on these applications.
