MSU Extension

The market for sweet cherries for the 2011 season is better than it has been in recent years. With the potential for good returns, growers should be diligent about American brown rot (ABR) control. Although we made it through the spring without frost incidents, growers have had other weather challenges that could predispose cherries to ABR infection. First, we have had perfect conditions for bacterial canker with our cool and wet spring, and canker is present in the majority of sweet cherry orchards across the state. Fruit with bacterial canker is now turning brown and much of it looks like it will not drop off during June drop – these fruits are particularly vulnerable to the ABR fungus.

Secondly, some blocks of sweet cherries received hail and we have observed sweet cherry fruit with varying degrees of damage from the hail. These fruits are also susceptible to ABR infection. Lastly, growers that had high levels of ABR in 2010 have a high inoculum load in the orchard and the potential for the fungus to reach epidemic levels this season. For instance, in ABR trials at the NWMRHS, where we had high levels of inoculum, we have mummies that are currently sporulating. Because of the aforementioned challenges this spring, we are recommending growers take precautionary measures to control ABR for the remainder of the season.

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 plums. 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. Under ideal conditions, sporulation can be initiated within three days after infection. American brown rot causes fruit rot before and after harvest, greatly reducing the quality and quantity of the yield, particularly in heavily bunching sweet cherry varieties.

Factors that contribute to ABR infection before harvest include warm, wet conditions as the fruit begins to ripen and increase in sugar content. The optimal temperature for infection is between 67°F and 77°F, and spore production is greatest between 59°F and 74°F. Although fruit injury may lead to increased infection, the ABR 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.

We have been experiencing warm and wet conditions this season – optimal for ABR infection and spread. These weather conditions are following 2010, where we observed significant ABR infection in many orchards throughout the state. Hence, there is a lot of fungal inoculum in many stone fruit blocks. One thing to remember about diseases like ABR is that when environmental conditions favor diseases, it is almost impossible to keep fruit from becoming infected. Also, the high susceptibility of sweet cherry cultivars to ABR infection is also an important factor.

The ABR fungus is a prolific sporulator; each infected fruit is a ready source of large numbers of new spores. As stated above, fruit infection to sporulation can occur in as little as three days; thus, it is critical to keep fruit surfaces covered when conditions are optimal for infection. Also, if growers are scouting and observe fruit infected with ABR, 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 by the fungus.

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

Fungicides. Fungicides with effectiveness against ABR fruit infection include the sterol inhibitor (SI) fungicides Indar, Elite, Orbit and Quash, the strobilurin fungicide Gem, and two fungicides containing two modes of action, Pristine (strobilurin and boscalid) and Adament (strobilurin and SI).

Of the SI fungicides, Indar has the highest efficacy against brown rot. The typical rate of Indar 2F used is 6 fl. oz. per acre. SI fungicides act in a quantitative manner, meaning that increased fungicide doses are more effective in control, and may also control fungal isolates that are developing reduced sensitivity. We are starting to observe a few ABR isolates in Michigan that are reduced in sensitivity to SI’s (see accompanying article, Current status of sterol inhibitor fungicides for control of American brown rot in Michigan).

We have obtained a Section 24(c) label for Indar allowing the use of increased fungicide rates of up to 12 fl. oz. per acre per application. Again, the normal field label rate is 6 fl. oz. per acre. Note that there is a maximum seasonal allowance of 48 fl. oz. per acre. The 6 fl. oz. per acre rate should provide excellent ABR control, provided fruit are effectively covered during the current and upcoming warm, wet ABR-conducive weather. However, growers that are concerned with possible population shifts could increase the rate of Indar used to 8 fl. oz. per acre. This is a 33 percent increase in fungicide rate and should be effective in controlling any fungal isolates with reduced SI sensitivity.

Of the other fungicides, Pristine is one that has been utilized in the southeastern United States where SI resistance in brown rot is prevalent. While not as effective as Indar, Pristine still has very good activity against brown rot. The highest label rate (14.7 oz. per acre) should be used.

Growers should take the time to scout their orchards for American brown rot. If the orchard has this disease present, it should not be difficult to locate fruit with the typical gray-brown fungus on the fruit. Blocks with bacterial canker, hail damage or problematic orchards should be the first stop in scouting. If ABR is detected, growers should move to an every row spray regime for fungicide applications, particularly if the trees are large. Growers should also slow down the tractor speed to obtain adequate coverage. Efforts should be made to apply fungicide applications with ample water to ensure that the entire tree is properly covered.

Controlling obliquebanded leafroller is also of utmost importance as we approach harvest. These larvae web cherry clusters together and prevent fungicide penetration inside the cluster. If growers know that they have had a problem in the past with obliquebanded leafroller and did not control them at the overwintering generation timing, they will decidedly need to apply an insecticide for the summer generation as these insects could ultimately impact ABR control at or near harvest.

Read the follow-up article, "Further thoughts on managing brown rot in sweet cherry for 2011".

Dr. Sundin's work is funded in part by MSU's AgBioResearch.