Tuesday, July 28, 2015

Late season disease management considerations

Some of our early cultivars such as Alandale is changing it color.  It is just a matter of time to see other cultivar to enter phase of veraison.  Hopefully, we will keep seeing dry weather with little bit of rain here and there....

There are several diseases to be considered at this stage.  One of them is Botrytis.  The spray timing for Botrytis is at bloom, bunch closure, and veraison.  The pathogen seems to be active throughout the season, but Botrytis likes berries when they get mature and accumulate more sugar.   As usual, canopy management is the first line of defense against grape disease management and this is particularly true for Botrytis since it tends to develop after a long period (15-17 hours) of high humidity condition.

Please keep in your mind that Botrytis is very well known for its capability to overcome fungicides.  Thus, a rotation of mode of action groups is very important aspect for Botrytis management.  The 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.  Two different fungicides with two different chemical names may have the same FRAC code; therefore, when you are in doubt, please make sure to rotate among the FRAC codes.  Moreover, many new materials have more than one modes of action together.  For instance, both Pristine and Luna Experience (or Tranquility) are mixed products and both contain SDHI (FRAC code = 7).  Thus, if you use Pristine for one spray, application of Luna Experience won't be considered as a rotation of mode of action.

These are some of chemical (and FRAC code) that you can use for Botrytis management: Elevate (17), Scala (9), Vangard (9), Switch (9+12), Inspire Super (9+3), Luna (Tranquility 7+9, Experience 7+3), Meteor/Rovral (2),  Endura (7), and Pristine (7+11).  Since captan has moderate efficacy against Botrytis, it is always a good idea to tank mix a Botrytis material with captan for resistance management.

If you think one of fungicides you are using is not providing sufficient control, please contact Dr. Baudoin or me.

It is also important to avoid risk of wounding 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 fruit skin is damaged.  Thus, management of insect such as grape berry moth, or birds, or powdery mildew at early in the season, can lower the risk of Botrytis development.  Moreover, sour rot pathogens also really like going after wounds; so, wound management would be the key for the sour rot management as well.  

Another diseases to be considered at this time of the year are ripe rot and bitter rot.  Both are considered as a warm season diseases, and 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.  You will see a round dark brown lesions on the top portion of an infected berry (as you see on the picture above); however, the difference between sun burn and ripe rot is that the tiny dots you will see within the lesion, which are fruiting bodies 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 many shriveled berries on a cluster.  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, Ziram, captan, and QoI (Strobirulin) fungicides are currently recommended.  Please rotate among these three mode of actions because some of isolates causing ripe rot are not sensitive to one of these materials.

Monday, July 13, 2015

You start to see black rot now, so, what should you do?

I have received several emails about black rot recently.  With the frequency of rains we observed so far, it is not surprising to see small amount of black rot even if you had a very good coverage of fungicide. Typically black rot starts off as brown discoloration on the berry, and associated with production of fruiting bodies which are often look like black dots (the size of ground black pepper?).  The picture above is not the best, but you see black dots on the one at the top of the picture.  Eventually, the infected berry will become hard and dry mummified berry with a lots of fruiting bodies, which you can see here and there in the picture.

If you are seeing something like the picture above, chances are the infection happened at least two weeks ago when berries were susceptible to the infection by black rot pathogen (and so as by powdery mildew or downy mildew pathogen).  I can say that because it takes about 2 weeks after infection for black rot pathogen to cause symptoms.  However, at this point of the season, your berries are matured and should be resistant to new infections.  Thus, spores from the infected berries should not cause further infection on healthy berries.  But I should note that if the vine had multiple infection events in the past, you may see increase in black rot.  Often time, infection happened at the end of the critical period for black rot tends to take time to show its symptom.  However, there is nothing we can do about the infected berries.

So, the question is what to do.  Since it should not cause further new infections on berries, I do not see the need for a black rot specific fungicide application.  However, if you insist, or if you have a late maturing varieties that may still be in the edge of the critical period, or more importantly, if you have a concern on ripe rot, you can include either a QoI (Abound, Flint, etc) or DMI (Rally, Elite, etc) to your next fungicide application.  Please note that captan won't be effective against black rot.  Unlike black rot, grape berries seem to be susceptible to ripe rot pathogen throughout the season. 

If you are considering removal of infected berries, you may want not only to remove them from the vine, but also take the infected berries out from your vineyard.  Even if you drop them on the ground, these infected berries will survive over the winter and become a source of spores in coming spring.  This pathogen can produce airborne spores, thus, spores on the ground can be blown off to leaves.  The mummies are small in size, thus, I doubt that a lawn mower will break them down.  Moreover, sending people to pick infected berry will cost you quite a bit. 

Thus, what I would do is just leave them as is.  The healthy berries won't get new infection, thus, I do not see the risk on yield reduction beyond what have been infected at this point.  By the time of harvest, the infected berries should become dry mummified berries that are easy to spot on a sorting table, and even if they are mixed into a must, this disease is not known to affect the quality of wine.

Lastly, if you had observed a fair amount of black rot this year, please make sure to have a good protection program next year, especially if it ended up like a rainy one like this year.