MSU Extension

Apple scab is a consistent disease problem year-to-year in Michigan orchards. Primary scab infection is initiated by ascospores produced in leaf litter from infected leaves that overwintered from the previous season. Controlling primary scab is critical for preventing fruit infections and keeping scab from becoming a full-season problem.

In many commercial blocks, scab infection levels of 1-5 percent at the end of the 2013 season signify that there is abundant inoculum present for this season. Rain events trigger spore release and wetting events of sufficient duration at particular temperatures result in scab infection events (see page 98 of the “2014 Michigan Fruit Management Guide” from Michigan State University Extension). Fungicide management of scab is best conducted when fungicides are present on trees prior to infection periods, and this is especially critical on highly scab-sensitive McIntosh.

The succinate dehydrogenase inhibitor (SDHI) class of fungicides is relatively new to Michigan and includes Fontelis (Dupont) and two premixes, Luna Tranquility (Bayer) and Merivon (BASF). Fontelis contains the SDHI compound penthiopyrad, Luna Tranquility is a premix of the SDHI fluopyram and the anilinopyrimidine (AP) pyrimethanil, and Merivon is a premix of the SDHI fluxapyroxad and the strobilurin pyraclostrobin. Each of these fungicides has strengths and weaknesses which I will cover below after outlining three general considerations for SDHI fungicide use.

General considerations for using SDHIs on apples

1. The best timing for use of the SDHIs is pink through bloom although applications as early as tight cluster can be used if powdery mildew is a persistent problem. All three of the SDHIs are very good to excellent fungicides for control of powdery mildew.
2. Restricting use of the SDHIs to two applications per season is a great strategy to balance current season control with long-term resistance management, i.e., limiting exposure of the scab and powdery mildew populations to SDHIs.
3. The potential for SDHI resistance development in the scab pathogen Venturia inaequalis looms down the road. Each of these fungicides, including the premixes, should always be tank-mixed with a protectant fungicide for resistance management purposes.

Fontelis

What I like most about Fontelis is that it is a single-ingredient fungicide, thereby giving the grower the flexibility to mix with whatever other fungicides are desired. We have tested Fontelis yearly in apple scab fungicide trials conducted at Michigan State University since 2006 and it has always provided very good to excellent scab control. The label rate for Fontelis range for scab is 16-20 fluid ounces per acre. The preferred protectant tank-mixing partner for Fontelis is an EBDC fungicide used at a 3-pounds-per-acre rate.

Fontelis contains an unspecified amount of mineral oil in its formulation. In 2013, isolated cases of phytotoxicity were reported in New York following the use of Fontelis plus Captan. However, to my knowledge, the actual cause of the phytotoxicity was not determined, and was likely due to specific combinations of Fontelis plus Captan plus whatever else was in the tank. The easiest solution to this issue is to tank-mix Fontelis with an EBDC exclusively. Since EBDCs have a 77-day preharvest interval (PHI), you would have to substitute Fontelis out of an application that late and potentially use a Merivon plus Captan application.

Luna Tranquility

Luna Tranquility is a premix of the SDHI fluopyram plus the AP fungicide pyrimethanil, which is sold as Scala. Luna Tranquility is a substitute fungicide in Michigan for Luna Sensation, used in the rest of the United States for apple scab, because we have widespread resistance to strobilurin fungicides in the scab fungus in Michigan. The availability of Luna Tranquility is good news, as this premix contains two fungicide components that are effective for scab control. However, remember that Scala, and AP fungicides in general, are most effective early in the season, and are less effective for fruit scab control. Therefore, the best timing for Luna Tranquility may be to begin use at tight cluster.

We’ve tested Luna fungicides – either Luna Sensation or Luna Tranquility – yearly for scab control since 2008 with consistently excellent results. The label rate range for Luna Tranquility for scab is 11.2 to 16 fluid ounces per acre. Again, tank-mix Luna Tranquility with an EBDC or Captan for resistance management and the protectants will also provide a boost in fruit scab control. Luna Tranquility has a PHI of 72 days, but I wouldn’t suggest using it that late in the season anyway.

Merivon

Merivon is a premix of the SDHI fluxapyroxad plus the strobilurin pyraclostrobin. Remember, the pyraclostrobin component of this premix has no utility for scab control as the large majority of the scab population throughout the state is resistant to strobilurins. Therefore, think of Merivon as a single ingredient SDHI fungicide.

We’ve tested Merivon yearly for scab control since 2010 with consistently excellent results. There is a wide label rate range on apples of 4 to 5.5 fluid ounces per acre. While my preference is to use a higher rate, at least 4.5 to 5 fluid ounces per acre, to maximize the length of time in years that we can use Merivon, we have not observed significant differences in scab control when comparing treatments using 4 or 5.5 fluid ounces per acre. Merivon should not be tank-mixed with oils as problems with phytotoxicity can result from that.

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