Finally, we are having relatively dry weeks! Most of us are about to finish critical time where clusters are susceptible to infection by downy mildew, powdery mildew, and black rot. This critical time varies by varieties, but in general 4 to 6 weeks and 3-4 weeks from bloom for V. vinifera and V. labrusca species, respectively. After this critical period, you should be able to relax a bit because these pathogens no longer able to cause disease on berries.
So, what’s next? As usual, disease dynamics really depends on environmental conditions, varieties grown, and cultural practice you employ, but in general, this is the moment when you will be thinking about late season diseases such as Botrytis, ripe rot, bitter rot, and sour rot.
The spray timing for Botrytis is at bloom, bunch closure, and veraison. The pathogen seems to be active throughout the season, and the reason we recommend application of a Botrytis specific material at bunch closure is that this most likely to be the last opportunity for you to deliver the material into the clusters, especially if you have tight cluster variety, such as Chardonnay. Also, please keep in your mind that Botrytis is very well known for its capacity to overcome fungicides. Thus, a rotation of mode of action groups is very important aspect for Botrytis management. Mode of action for a particular fungicide can be found as a FRAC code, which you should be able to locate on the label. For example, Rovral belongs to FRAC code 2. When you are in doubt, please rotate the FRAC code because two different fungicides with two different chemical names may have the same FRAC code. For instance, both Pristine and Luna Experience contain SDHI (FRAC code = 7). If you think one of fungicides you are using is not providing sufficient control, please contact Dr. Baudoin or me.
Another management tip for Botrytis is about wounding on berries. Although Botrytis is capable of causing disease by penetrating grape’s fruit skin tissues, it prefers going after wounds. Also, at bloom infection by Botrytis tends to develop into disease when grape fruit skin is damaged. Thus, management of insect such as grape berry moth, or birds, or powdery mildew at early in the season, can results in the management of Botrytis. Moreover, sour rot pathogens also really like going after wounds; so, wound management would be the key for the sour rot management as well. If you have not, please refer to Dr. Pfeiffer’s recent notes about late season insect management that he sent out in July 1st.
Both ripe rot and bitter rot are considered as a warm season diseases, and in fact, people in the south tend to suffer more from these diseases; however, we can find ripe rot in Northern VA year after year. In addition, I have heard from growers from PA and MD that they are having issues with ripe rot as well. Unfortunately, ripe rot in particular, has been misdiagnosed more than other rots. It shows up as if the berries are sunburned, thus you will see a round dark brown lesions on the top portion of an infected berry. The difference would be the tiny dots you will see within the lesion, which is fruiting body of the pathogen. As the time progresses, the lesion expand into the whole berry, and infected fruits become shriveled raisin-like fruits. With a severe infection, you may see majority of berries on a cluster shriveled down. Unlike sunburn, it does change the taste of the fruit and wine. Unfortunately, the change is in the negative direction, and a study showed that a consumer panel could detect only 3% contamination in the mast.
Both ripe rot and bitter rot seem to be able to infect berries from time of flowering to the harvest. Thus, you need to protect berries from infection, especially if you have experienced issues with these diseases in the past. Mancozeb, captan, and QoI (Strobirulin) fungicides are currently recommended. Please rotate among these three mode of actions because some of isolate causing ripe rot are not sensitive to one of these materials.