Control of Postharvest Decay with Chemical and Biological Fungicides
During the last few years we have used the registered biocontrol agents BioSave 110 and Aspire (now called I-182) for control of blue mold and other postharvest diseases of pear fruit in a variety of commercial and laboratory tests. Generally, the use of these agents has been beneficial to decay control, although the level of performance has been quite variable, and typically inferior to that of the fungicide thiabendazole (TBZ). It is important then to ask whether or not use of these biocontrol products is worthwhile.
First, it should be mentioned that it is not uncommon among biocontrol agents under investigation for control of various diseases of agricultural crops to provide control at levels of 40 to 60%, while chemical controls used against susceptible populations of disease pathogens may provide 80 to 99% control. So the situation in which we find ourselves in seeking improved control of postharvest fruit decay is not unique. The value of introducing biocontrol agents in pome fruit decay management strategies does not depend on being able to out-perform fungicides in head-to-head tests with susceptible pathogen populations.
Currently registered biocontrol agents can best benefit pear packers by using them in combination with TBZ. Here are four reasons why biocontrol agent combinations with TBZ can be beneficial:
- Fungicides typically perform best when they are
used to control pathogen populations which are low in
either number or strength. Accordingly, biocontrol agents
which interfere with pathogen ability to colonize fruit
wounds or compete with the pathogen for nutrients can allow
more fungicide to be available for direct control of
pathogens. The biocontrol agents are lowering the effective
"pressure" on the fungicide.
- Because of the situation described above, it may
be possible to lower rates of fungicide when combined with
a biocontrol agent, and still achieve the level of control
expected of the fungicide alone at maximum rate. The
virtues of this depend on the market strategies and
intended destinations for the treated fruit.
- TBZ, like many fungicides, is limited in the range
of pathogens which it is able to control. Several species
of pome fruit pathogen which at times cause significant
levels of decay are not sensitive to TBZ. Application of
biocontrol agents can broaden the spectrum of protection
against the range of pathogens with which stored fruit
contend, albeit protection is best when both the fungicide
and biocontrol agent are effective.
- Just as some pathogen species are not sensitive to TBZ, strains which are TBZ-resistant exist within common species which are otherwise TBZ-sensitive. A biocontrol agent-TBZ combination can broaden the range of protection to at least partially control resistant strains. It is also possible that by lowering the "pressure" on the fungicide (as described in reason 1 above), the increase of resistant populations can be delayed.
In addition to comparisons of biocontrol agents and fungicides, we recently conducted an experiment to determine appropriate timing of application of TBZ and BioSave 110 when the two products are applied at different times. Either product was applied to Bosc pears immediately after harvest, then the other product was applied six weeks later. Treatments also included no treatment at harvest, no treatment six weeks later, or no treatment at all. The best decay control was observed when TBZ was applied immediately after harvest, and BioSave 110 was applied to the same fruit six weeks later. This was better than when the products were applied in the reverse order.Similarly, an experiment comparing the effectiveness of TBZ when applied immediately after harvest or 3, 6, 9, or 12 weeks later showed that control was best when TBZ was applied early in the storage period. Approximately 6 to 9 weeks after harvest, in this experiment, the benefit obtained from TBZ treatment began to decrease. Thus both of these experiments emphasize the value of early treatment with TBZ.
Oregon State University, Southern Oregon Research and Extension
13th Annual Postharvest Conference