Control of post-harvest fruit rots in blueberries
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Fruit rots in blueberries, such as anthracnose fruit rot (Colletotrichum acutatum) and Alternaria fruit rot (Alternaria spp.), are generally separated into two types: field rot and post-harvest rot. The former can be seen on berries in the field before harvest and is especially common when berries are left on the bushes too long. So timely harvesting is an important control measure. Post-harvest rot can develop on sound-looking berries, as spores from infected berries can infect them in the field before or during harvest or during processing. Often, these berries look healthy at harvest, but start to rot soon after. Rot may be slowed down by refrigerated storage, but will resume on the supermarket shelves, lowering fruit quality. These infections can also contribute to high microbial counts in frozen berries, leading to rejection of fruit lots by some buyers. Rapid cooling of harvested fruit is important in reducing post-harvest fruit rot incidence, particularly at the later harvests when disease pressure is generally high.
While fruit rot is often not visible until the berries ripen or even after harvest, it is prudent to assume that you will have a fruit rot problem if you had problems in past years. If the first blueberries are starting to show rot, fungicide sprays can still limit new infections of neighboring healthy berries. Applications within one to two weeks of the first harvest can still be beneficial in preventing these late infections. In fact, an additional fungicide application between the first and second harvest may be beneficial under high disease pressure.
Examples of fungicides that can be used during fruit development and ripening are discussed below. The strobilurins (Abound, Cabrio, Pristine) are all highly effective against anthracnose with Pristine having the most broad-spectrum activity since it contains two different active ingredients. However, it is also the most expensive of the three. Pristine will also have excellent activity against Phomopsis, while Cabrio has good and Abound fair activity against this disease. All are supposed to have moderate to good activity against Alternaria fruit rot and become quickly rainfast since they are locally systemic. Switch (cyprodinil and fludioxonil) also has some systemic properties and provides simultaneous control of anthracnose, Alternaria, and Botrytis fruit rots. Thus it may be a good choice if several fruit rots are a concern. Captevate (captan and fenhexamid) at the high rate will provide good control of anthracnose as well as Botrytis fruit rot, but this disease tends to be less common in Michigan. Captevate is also fairly expensive. Aliette (fosetyl-Al) is a highly systemic fungicide that provides good control of anthracnose, Alternaria fruit rot, and Phomopsis. Of course Topsin M + Captan can still be used, provided the 7-day PHI of Topsin M is taken into consideration. While Topsin M is a systemic material and is more active against Phomopsis, Captan as a protectant will do much of the work against anthracnose. Therefore, if anthracnose is the disease you wish to control and the weather is relatively dry, a Captan or Captec spray alone may suffice. Do take note of the pre-harvest intervals for the various fungicides.
Dr. Schilder’s work is funded in part by MSU‘s AgBioResearch.