From what I have been told, there may be a shortage of oxytetracycline products this year, due to the use of this antibiotic in Florida for potential control of citrus greening. All of this is occurring at a time in which we can expect ideal conditions for bacterial spot development (warm to hot and wet at petal fall and shuck split and shortly thereafter). For susceptible varieties, this means we will likely have to fall back on copper for use in the cover sprays, despite the potential for leaf damage.
Based on trials conducted by Dr. Harald Scherm and others, the use of copper at 0.5 oz metallic copper equivalent (MCE) (0.03 lb MCE) per acre provided equivalent control of bacterial spot to that of oxytetracycline. I later conducted a trial to determine how much damage would occur if 0.6, 1.2 and 2.4 oz MCE per acre is utilized weekly during cover sprays; damage levels were acceptable at the low rate of 0.6 oz MCE (minimal damage and premature defoliation), but increasing levels of 1.2 and 2.4 oz MCE were too damaging for sustained use. Another caveat is that we only applied copper six times in this phytotoxicity trial, whereas one might have to apply quite a few more applications for a full-season cultivar like ‘O’Henry’. If it is dry and projected to be dry during any part of the cover-spray timeframe, you could stretch the interval between copper applications to take advantage of a dry period. However, you never want to be “naked” when a rain event occurs; in other words, you would want copper to be applied ahead of a rain or significant dew event to provide good coverage and protection against bacterial infection.
Careful calibration and correct rates are important to avoidance of damage when applying copper. The rates of most coppers are included in the IPM guide, but I have reconfigured some of the more commonly utilized products in the table below for your convenience. I would personally shy away from the higher rates listed in the IPM guide for cover sprays, and the 0.13 lb/100 gals or 0.08 lb/60 gals Kocide 3000 rate would be my choice for copper rates during that timeframe; likewise, the 6 fl ozs/100 gals or 3.6 ozs/60 gals Copper Count N would be acceptable. Generally, a range of 0.5 to 0.75 oz MCE per acre is a good target for the cover spray copper applications.
I asked Dr. Dave Ritchie (North Carolina State) for his input and review of the above information, and he had the following additional comments:
1. Copper is only protective (as pointed out).
2. Once bacterial spot gets started, only a change in weather conditions will stop it without previous controls having been applied.
3. Injury to foliage WILL occur if an effective antibacterial rate is being used and sprays should be pushed to that point. HOWEVER, and the following is EXTREMELY important — before every spray, the orchard should be inspected to determine if that same rate is appropriate to use again or does it need to be reduced, spray intervals should be by label (not closer than 5-7 days).
4. Copper can accumulate on foliage if adequate rainfall is not occurring to cause some wash-off. It is this cumulative effect that seems to lead to injury, and there is a “tipping point” that an additional small amount results in jury.
5. Once leaves become mature, they are much more tolerant of copper.
6. Spray copper, but don’t excessively spray!!!
7. Growers should monitor the weather. Rainfall — even with cool temperatures — is the primary driver for infection, and 12-18 h of misty conditions are more conductive than a heavy rain for an hour or so.
8. Shuck split begins the critical susceptible fruit stage.
Dave also states, “I have been monitoring spring cankers the last few weeks, and they are “loaded” with pure cultures of Xap bacteria! Just waiting for moisture to wash the bacteria.”
| Copper Formulations | Petal Fall to 1% Shuck Split | Shuck Split to 10% Shuck Off | 7 to 10 Days after Shuck Split Spray | Cover Sprays |
| Nordox 75WG | 0.67 lbs/100 gals | 0.2-0.25 lbs/100 gals | ||
| (0.5 lbs MCE) | (0.15-0.2 lbs MCE) | |||
| Kocide 3000 | 0.25-0.5 lb/100 gals | 0.25-0.5 lb/100 gals | 0.13-0.5 lb/100 gals | 0.13-0.5 lb/100 gals
or 0.08-0.3 lb/60 gals |
| (0.075-0.15 lb MCE) | (0.075-0.15 lb MCE) | (0.04-0.15 lb MCE) | (0.04-0.15 lb MCE) | |
| Copper Count N | 8 fl ozs/100 gals | 8 fl ozs/100 gals | 4-6 fl ozs/100 gals | 4-6 fl ozs/100 gals or 2.4-3.6 ozs/60 gals |
| (0.05 lb MCE) | (0.05 lb MCE) | (0.025-0.0438 lb MCE) | (0.025-0.04 lb MCE) | |
| Comments | Regardless of the copper formulation used, the rate of MCE at petal fall should not exceed 0.50 lb MCE /acre, preferably in 100 gals. If mild foliar injury occurs, use lower rates. If injury is deemed unacceptable, do not use copper.
Coppers are concentration-sensitive materials. Rates are typically expressed as amount of formulated copper product/100 gals/acre. If gals sprayed/acre are reduced after shuck split/shuck off, it is important to reduce copper rates proportionately, otherwise phytotoxicity is likely to increase. Copper antibacterial activity and phytoxicity are pH sensitive. Coppers applied in spray tank water with pH less than 6.5 may have increased risk for phytotoxicity.
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Regardless of the copper formulation used, the rate of MCE should not exceed 0.25 lb MCE /acre, preferably in 100 gals. If mild foliar injury occurs, use lower rates. If injury is deemed unacceptable, do not use copper.
Coppers are concentration-sensitive materials. Rates are typically expressed as amount of formulated copper product/100 gals/acre. If gals sprayed/acre are reduced after shuck split/shuck off, it is important to reduce copper rates/concentrations proportionately, otherwise phytotoxicity is likely to increase. Copper antibacterial activity and phytoxicity are pH sensitive. Coppers applied in spray tank water with pH less than 6.5 may have increased risk for phytotoxicity. |
Regardless of the copper formulation used, the rate of MCE should not exceed 0.15 lb (2.4 oz) MCE /acre, preferably in 100 gals. If mild foliar injury occurs, use lower rates. If injury is deemed unacceptable, do not use copper.
Coppers are concentration-sensitive materials. Rates are typically expressed as amount of formulated copper product/100 gals/acre. If gals sprayed/acre are reduced after shuck split/shuck off, it is important to reduce copper rates/concentrations proportionately, otherwise phytotoxicity is likely to increase. Copper antibacterial activity and phytoxicity are pH sensitive. Coppers applied in spray tank water with pH less than 6.5 may have increased risk for phytotoxicity. |
Regardless of the copper formulation used, the rate of MCE should not exceed 0.15 lb (2.4 oz) MCE /acre, preferably in 100 gals. If mild foliar injury occurs, use lower rates. If injury is deemed unacceptable, do not use copper.
Coppers are concentration-sensitive materials. Rates are typically expressed as amount of formulated copper product/100 gals/acre. If gals sprayed/acre are reduced after shuck split/shuck off, it is important to reduce copper rates/concentrations proportionately, otherwise phytotoxicity is likely to increase. Copper antibacterial activity and phytoxicity are pH sensitive. Coppers applied in spray tank water with pH less than 6.5 may have increased risk for phytotoxicity. |
