Resistance management for postharvest fungicides is more difficult than it is for foliar fungicides.This is due to the fact that resistance management for postharvest fungicides must include what happens both in the field and in the packinghouse. Most postharvest diseases begin in the field and therefore the management practices performed in the field will have a direct bearing on the selection pressure that is imposed on pathogens entering the packinghouse. Resistance management in the field and in the packinghouse can not be mutually exclusive. With rare exception, there is a complete disconnect between the two. Growers do not consider what fungicides will be used in the packinghouse in making their field management decisions and packers do not change their practices due to what a grower uses in the field. As there is no easy solution to this problem, the following PACK RULES are the "best practice guidelines" for resistance management of postharvest fungicides.
Excellent resistant management in the field must include the following:
Prevent disease establishment by using effective field fungicide programs
Avoid use of fungicides with related chemistries to fungicides that might be used in the packinghouse.
Cultural practices are critical. Avoid fruit bruising or other injuries during harvest & transport.
Keep it clean. Sanitize to reduce inoculum entry into the packinghouse.
Use only clean bins & wash them between loads
Never return culls from the packinghouse to the field as this can introduce potentially resistant spores into the orchard.
By following thePACKyou essentially do the following:
Reduce disease development & the introduction of inoculum
Avoid imposing a selection pressure for resistance development to a fungicide that will be used in the packhouse
Reduce fruit susceptibility to decay pathogens, the introduction of spores into the packinghouse, and the return of resistant spores to the field.
For excellent resistant management in the packinghouse follow these RULES:
Rotate between different fungicide classes or use mixtures prior to the development of resistance.
Use labeled rates (do not cut rates as this imposes sub-lethal selection pressure on fungal populations).
Limit the total number of fungicide applications of any one class to less than 4 per season.
Educate yourself about fungicide activity, mode of action, and class.
Start a fungicide management program when possible with multi-site mode of action fungicides.
Please note that if growers do not follow the PACK strategy then R and L of the RULES must take into consideration both pre and postharvest fungicides.
The following tables are a quick reference for cross-resistance between pre and postharvest fungicides.
*Fungicide groups are designated by FRAC (Fungicide Resistance Action Committee) base fungicides with similar modes of action or potential for cross resistance. See www.frac.info for more details.