Peach Disease - Bacterial Spot Differentiation from Copper Injury

It can be confusing to discern between bacterial spot disease and copper injury. This article describes offers guidance to avoid the pitfalls of using copper for disease control.
Peach Disease - Bacterial Spot Differentiation from Copper Injury - Articles

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Bacterial spot on peach fruit. Photo by S. Bardsley.

Over the last several weeks, many folks have been spraying copper to combat bacterial spot on peach and nectarine. Not only have we had excellent bacterial spot conditions these last several weeks, but the conditions have been slow drying thereby exacerbating the effects of copper phytotoxicity.

This is a very important aspect to embrace.

I have been fielding many questions recently from those who are having trouble discerning the difference between copper injury and bacterial spot and, as result, offered a 1 hour workshop on how to tell the difference between the two types of symptoms. For those who were unable to attend, this is a summary of what was discussed; for those who attended, this is a healthy review. In addition, please see the picture for a side-by-side comparison of a leaf with bacterial spot versus a leaf with copper injury.

Important nuggets of wisdom to memorize when using copper on stone fruit trees

  • Copper is a general biocide: Copper will kill fungi, bacteria, AND plant cells.
  • The leaves of stone fruit trees are more sensitive to copper phytotoxicity than apple leaves.
  • Stone fruit varieties can vary in sensitivity to copper (some more sensitive than others).
  • Do not tank mix copper with a foliar fertilizer or phosphorus acid product (i.e., Rampart, Phostrol, Prophyt).
  • Captan and sulfur will also cause phytotoxicity (symptoms will be similar to copper injury).
  • On stone fruit: Biocides do not minimize the phytotoxic effects of copper when tank mixed (confirmed by Penn State and Virginia Tech). This is contrast to what some have seen when applying these mixes on apples.

How copper works: Fixed copper

  • Fixed copper is safer for plant tissue than "bluestone" copper.
    May be used throughout the growing season, but will cause some level of leaf damage.
  • Low solubility in water, resulting in a lower risk for phytotoxicity. High rates of fixed copper may cause plant damage, however.
  • When sprayed, the particles will persist on the plant surface after the spray dries; copper ions are gradually released from copper deposits each time the plant surface becomes wet (with water/rain).
  • The gradual release of copper ions from the copper deposits provides residual protection against plant pathogens. Copper sprays act as protectant fungicide/bactericide treatment, but lack post-infection activity.
  • Copper phytotoxicity worsens under slow drying conditions. Slow drying time (e.g. rainy conditions) increases solubility of copper, release of copper ions, and thereby phytotoxicity of copper fungicides.
  • Copper sprays will become more phytotoxic if they are applied in an acidic solution: beware of adjuvants, phosphorus acid fungicides, and mancozeb that lower pH of tank mixes. Growers may add lime to reduce potential for plant damage.
  • Common forms of fixed copper fungicides include: basic copper sulfate (Cuprofix, Basicop), copper hydroxide (Kocide, Champ), copper oxychloride sulfate (C-O-C-S), cuprous oxide (Nordox), and copper octanoate (Cueva)

How copper works: Bluestone (Copper sulfate pentahydrate)

  • Use as dormant spray, only.
  • Highly soluble copper ions can be phytotoxic to exposed plant tissue.
  • Often combined with lime to help "tie up" copper ions and slow their release.
  • No residual activity. Copper ions are released rapidly upon application (in contrast to fixed coppers). Common brands of copper sulfate include: Mastercop and Phyton

Bacterial spot leaf symptoms

Bacterial spot symptoms are always angular lesions. They are angular lesions because the lesion is bordered by the leaf's veins. Often times, you will see lesions along the midrib of the leaf; the tip of the leaf; and along the edges (similar to how water runs off of the leaf or settles - this is where the bacteria has the potential to accumulate and cause cell death). There may be a few lesions; there could be many lesions on a leaf (see picture). The leaf will eventually turn yellow and fall off the tree. It does not take many lesions on the leaf for the leaf to fall off. Lesions can occur on older and younger leaves.

Copper injury symptoms

The lesions resulting from copper injury are always round, circular lesions. Unlike bacterial spot, copper injury always has a lot of round, circular lesions on the leaf (see picture). The reason there are a lot of lesions is due to spray pattern that occurs on the leaf. The pattern of the lesions is random (as opposed to following the veins like bacterial spot). With continued use of copper, the older leaves will often develop a buildup of copper if rain does not wash off copper in between sprays.

This will often result in premature defoliation of older leaves since the phytotoxicity will increase with increasing amounts of copper. In contrast, younger leaves will typically look "much healthier" than older leaves since they have less residual copper.

Bacterial Spot: Note shape and appearance of lesions

  • Angular
  • Always bordered by the veins
  • Can have few or many lesions on a leaf
  • Yellowing associated with lesions
  • Leaf will always turn yellow then fall off (see leaf on left) - it does not take many lesions for this to occur

Copper Injury: Note shape and appearance of lesions

  • Round, like a water droplet
  • Randomly located on the leaf: like a spray pattern
  • A lot of "holes" present (remember: spray pattern)
  • Yellowing not always associated with lesions
  • Sometimes the leaf will turn yellow and fall off - severe damage

Minimizing copper injury on stone fruit

If there is a forecast where it will rain every other day for three weeks, it's best to minimize your use of copper to manage bacterial spot. Hindsight is always 20/20. During such wet conditions, use oxytetracycline (as long as you are not within the 21 day PHI) tank mixed with a biocide, such as Double Nickel or Serenade Optimum.

Last year, we saw bacterial spot control using Serenade Optimum (14 oz/A) in rotation with copper; the results were similar to when we used copper for every spray in the season. Oxytetracycline will only give about two days of control, but the biocide will give you a few more. This mixture can be applied every 7 days. If you choose to use a copper, be mindful of the spray interval: do not shorten the spray interval less than 7 days.

The popular copper, Cueva, says plainly on the label: Do not reapply within 7 days. With that said, the issue of alternate row middle sprays will come up and what to do. Keep in mind: sprays drift and some trees may be receiving higher amounts over a period of days due to alternate row middle sprays, thereby increasing the chance of phytotoxicity.

Bacteria are such a different beast compared to fungi and management tactics have to accommodate that fact. When it comes to bacterial diseases, I always advocate complete sprays. I know this may not be feasible for many; however, it is ultimately up the grower to weigh the pros and cons for what will in the end, be the most economical for their operation.

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