Control of brown rot of stone fruit is becoming increasingly unsustainable with the exclusive use of fungicides, especially during wet growing seasons. (Articles continues from last month).
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Chemical control still the key component of IPM
The stone fruit industry has relied on fungicides applied during bloom and fruit development for managing brown rot. However, yield losses still occur despite intensive application of fungicides.
Examination of grower spray programs from several orchards indicates that failures to control brown rot are due in part to leaving trees unprotected during wet events (infection periods) when nectarine and peach crops are highly susceptible to M. fructicola infection.
Timing of sprays
The time of application of both protectant and post-infection fungicides can be greatly improved by using infection period warnings in combination with information on other factors that influence infection risk such as crop susceptibility.
There are other reasons which could be affecting fungicide efficacy; these include poor application (e.g. coverage), loss of fungicide sensitivity and reduced persistence (e.g. wash-off during heavy rain), type of spray program (i.e. calendar vs. mixture protectants and curatives), and spray intervals especially during protracted wet periods when trees are growing fast under high inoculum levels.
Rain
The effect of heavy and frequent rain on fungicide persistence may explain poor control achieved in recent wet seasons where spray diary information indicated that fungicides were applied during all important infection periods.
All these issues highlight the importance of proper selection of fungicide and spray intervals according to the level of infection risk and the intensity of rain.
Fungicide selection
Worldwide, many fungicides are available for brown rot control, but few are highly effective against brown rot in the field.
Some of the most efficacious fungicides belong to the demethylation inhibitors (DMIs), dicarboximide, anilinopyrimidines, and the quinone outside inhibitors (QoIs) fungicide activity groups.
In Australia, only a very limited number of fungicides with good systemic (eradicant) activity against Monilinia spp. are currently registered for brown rot control (http://services.apvma.gov.au/PubcrisWebClient/welcome.do).
Propiconazole (DMI) and iprodione (dicarboximide) are systemic fungicides which have been used intensively in stonefruit orchards for many years. They also have some protectant activity, and iprodione can also be used postharvest.
The anilinopyrimidines cyprodinil and pyrimethanil have been successfully used to control brown rot in California but are reported to underperform in moist and warm climates of south-eastern United States.
They have been shown to have both protective and curative properties.
Going on overseas experience, the Australian industry could benefit from an increased arsenal of good systemic fungicides with curative activity. These ideally should be highly effective against blossom blight and brown rot, but less toxic to beneficials and able to be used close to harvest when the fruit is most susceptible to infection.
Resistance management is vital
Another factor that may influence the efficacy of fungicides is the presence of Monilinia spp. strains resistant to fungicides used in stone fruit orchards.
Field resistance in fungal pathogens can build up slowly (quantitative resistance) as was the case with DMI fungicides and the apple scab pathogen.
Field resistance can also develop within a few years of market introduction, as was the case for the benzimidazoles (i.e. benlate) leading to complete resistance (qualitative resistance).
Benomyl-tolerant isolates of M. fructicola were identified as early as 1976 from a stonefruit orchard in New South Wales, very soon after its introduction.
Overseas, strains of Monilinia spp. resistant to the dicarboximides (i.e. iprodione) the DMIs (propiconazole and fenbuconazole), MBC (e.g. thiophanate-methyl), the QoI (e.g. strobilurins) and SDHIs have already been reported.
This raises serious concern about the sustainability of fungicides prone to resistance development currently used for brown rot management in Australia, where very little work has been done to monitor resistance.
Sensitivity to fungicides
A recent in vitro study (Project MT08039) provided some insights into the sensitivity of M. fructicola isolates from stone fruit orchards in Victoria to propiconazole, iprodione, the MBC thiabendazole and the new fungicide fludioxonil.
As expected, all Monilinia isolates were sensitive to low concentrations of fludioxonil, a product mostly used as a post-harvest treatment in the USA.
However, some Victorian isolates were tolerant to high concentrations of iprodione and thiabendazole, with an isolate found to be highly resistant to thiabendazole, a fungicide no longer registered for stone fruit.
Resistance programs
New cost-effective tools are required to assess Monilinia spp. sensitivity to fungicides at the orchard level to help growers improve selection of fungicides.
In the meantime, to minimise the risk of resistance development, it is important to follow recommended fungicide resistance management strategies (http://www.croplifeaustralia.org.au).
In regions of the world where fungicide resistance in brown rot and other pathogens is a problem, the aim of anti-resistance measures is to slow the build up of resistant strains and to control resistant strains using other fungicide groups.
In some of these regions, brown rot management programs that use protectants with QoIs, DMIs, and dicarboxamides when needed is the preferred approach to control brown rot while maintaining pathogen sensitivity to each of the three site-specific fungicide groups.
Avoid Carpophilus damage
In southern Australia, Carpophilus spp. are major pests of stone fruit including cherries.
Economic losses of up to 30% have been reported at harvest due to direct consequence of Carpophilus damage.
Carpophilus beetles have also been implicated as vectors of M. fructicola spores. Anecdotal evidence suggests that controlling Carpophilus reduces the brown rot incidence in fruit. However, little work has been published to demonstrate the link between beetle populations with disease risk.
Project MT08039 has demonstrated that Carpophilus beetle populations can be lowered using the attract-and-kill (A&K) system which in turn reduced the incidence of brown rot.
However, the A&K system can only be effective if the Carpophilus population is at a medium to low level and it may take more than three seasons of trap deployment to achieve a low beetle population.
More research is needed to better understand the relationship between Carpophilus population and brown rot spread and severity to guide a more rational control of both problems.
Bottom line: integrated control the key
Chemical control is still a key component of IPM, however, because the brown rot pathogen is adapted to infect stone fruit throughout the season, it is best managed using an integrated approach that includes sanitation measures and infection risk prediction for scheduling preventive and post-infection fungicide applications.
The monitoring of infection periods is a key element of infection risk prediction and best management practices to ensure systemic post-infection treatments are only used when necessary, minimising the risk of resistance development.
Fungicides with curative and some protectant activity should be used if possible preventively to stop spores from infecting susceptible tissue. This is because applying such fungicides to suppress established fungal infections increases the risk of resistance development.
It must be remembered that fungicides are only tools and in reality what affects the success of brown rot control with fungicides depends on orchard inoculum levels, spray timing in relation to stage of crop susceptibility, fungicide mode of action, the spray strategy, and then everything else.
In the absence of information on sensitivity of Monilinia spp. populations to site-specific fungicide groups prone to resistance development such as the DMIs, fungicide resistance management strategies should be used to ensure the efficacy of site-specific fungicides is not lost through resistance.
The prediction of infection risk can be greatly enhanced by integrating information on orchard factors that influence infection risk such as inoculum potential, stages of crop susceptibility and Carpophilus beetles trapping data in addition to knowledge of disease cycles and fungicide efficacy.
The integrated approach should also include assessing rot potential at harvest and practices which minimise postharvest losses due to fruit rots.
For more information and images, see Tree Fruit Dec 2012
Published, with thanks, from Australian Stone Fruit Grower
