MSU Extension

American brown rot is caused by the fungus Monilinia fructicola. This fast-growing fungus is an important pathogen on cherries (sweet cherries in particular), peaches, apricots, nectarines and plum. The fungus attacks fruit, blossoms, spurs and shoots; under ideal infection conditions, the fungus can rot individual cherry fruit within 24 hours (see photo). The fungus sporulates from infected fruit, continually increasing inoculum for further infections to other fruit on the tree. Under ideal conditions, sporulation can be initiated within three days after infection. American brown rot causes fruit rot before and after harvest, greatly reducing quality and quantity of the crop, particularly in heavily bunching sweet cherry varieties.

Factors that contribute to American brown rot infection before harvest include warm, wet conditions as the fruit begin to ripen and increase in sugar content. The optimal temperature for infection is between 67 and 77 degrees Fahrenheit, and spore production is greatest between 59 and 74 F. Although fruit injury may lead to increased infection, the American brown rot fungus can cause infections when no wounds are present. Other factors influencing increased infection are fruit-to-fruit or fruit-to-branch contact on trees.

The American brown rot fungus is a prolific sporulator; each infected fruit is a ready source of large numbers of new spores. As stated above, initial fruit infection to sporulation occurs in as little as three days; thus, growers need to keep fruit surfaces covered when conditions are optimal for infection. Also, if growers are scouting and observe fruit infected with American brown rot, it is likely that there are many other fruit that are infected, but not showing symptoms yet. It is not possible to stop brown rot infections on fruit once they are initiated; however, fungicide applications will protect other fruit that have not been infected.

The two most important issues in American brown rot control of fruit infection are use of an effective fungicide and fungicide coverage of fruit surfaces.

Although this article focuses on the succinate dehydrogenase inhibitors (SDHI) fungicides, there are three main classes of fungicides with effectiveness against American brown rot:

1. Sterol inhibitors (SIs): Indar, Elite, Orbit, Quash
2. Succinate dehydrogenase inhibitors (SDHIs): Merivon, Luna Sensation, Fontelis
3. Strobilurins: Gem

The SIs, Indar in particular, have been the most effective fungicide class to protect fruit from American brown rot since the late 1980s. However, based on orchard surveys conducted over the previous three years, our results have shown that the sensitivity of the M. fructicola isolates to Indar has been declining. Fungal sensitivity or resistance to SI fungicides acts in a quantitative manner, meaning that a fungus with a decreased sensitivity to one rate can be controlled by a higher rate of the same fungicide. Results from orchard surveys conducted by the Sundin lab from 2007-2013 have indicated that there is clear shifting in the American brown rot population, and that approximately 5 percent of the American brown rot isolates in Michigan are resistant, and 50-75 percent of all isolates are shifted, which indicates reduced sensitivity. Our work has shown that the 12 fluid ounce per acre rate of Indar does not affect many of the SI-resistant isolates at all; at best, this rate of Indar will only limit growth of reduced sensitive isolates for four to five days maximum.

Thus, to ensure effective control of American brown rot this year, especially on sweet cherries and peaches, if SIs are used they should be rapidly alternated with fungicides from other classes, including Merivon, Luna Sensation or Gem. All American brown rot applications should be tank-mixed with Captan for resistance management as Captan has some activity against American brown rot and will be critical for protection of the SDHIs and strobilurin fungicides from future resistance development.

Merivon

Merivon is a premix of the SDHI fluxapyroxad and the strobilurin pyraclostrobin and has shown excellent activity against fruit American brown rot in tests conducted on peaches in orchards in Georgia with SI-resistant M. fructicola. The rate range on the label for Merivon is broad, 4 to 6.7 fluid ounces per acre. In studies conducted in Georgia and North Carolina, similar results were obtained using 5.5 or 6.5 fluid ounces per acre. For Michigan in 2014, a tank-mix of Merivon at 5.5 fluid ounces per acre plus Captan 80WDG at 2.5 pounds per acre should provide excellent control of fruit brown rot.

Luna Sensation

Luna Sensation is a premix of the SDHI fluopyram and the strobilurin trifloxystrobin. Trifloxystrobin is also sold individually as Gem. Luna Sensation has also shown excellent activity against fruit American brown rot and should be used in a tank-mix with Luna Sensation at 5.5 fluid ounces per acre plus Captan 80WDG at 2.5 pounds per acre. Luna Sensation is not registered on peaches.

Fontelis

The active ingredient of Fontelis is the SDHI penthiopyrad. Fontelis has good activity against American brown rot fruit infection and should be used at the high label rate of 20 fluid ounces per acre plus Captan 80WDG at 2.5 pounds per acre. Michigan State University Extension reminds growers that on cherries, Fontelis has no activity against cherry leaf spot.

Summary

Staying ahead of fruit brown rot infection means keeping fruit surfaces covered with effective fungicides, especially when weather conditions favor American brown rot infection, as we have seen for the majority of the 2014 growing season. Use the best fungicide combinations, Merivon or Luna Sensation + Captan, on the most susceptible crops, such as sweet cherries and peaches. For less susceptible crops such as tart cherries, Indar at 12 fluid ounces per acre plus Captan application may be more effective. Gem plus Captan can also be used.

The 24(c) SLN label for Indar calls for applications at seven- to 10-day intervals. Thus, an alternative fungicide must be applied within four to five days of an Indar application, particularly if weather conditions are warm – highs in the 70s, low 80s – with rain. The best alternative to Indar for American brown rot fruit rot control are the new SDHI premixes Merivon and Luna Sensation. An alternating fungicide approach of Indar plus Captan, SDHI plus Captan, Indar plus Captan, SDHI plus Captan would represent our current, best strategy for American brown rot control. Also, the use of the SDHI as the last spray before harvest would ensure the longest effective control of American brown rot post-harvest.

Dr. Sundin and Rothwell’s work is funded in part by MSU’s AgBioResearch.