PPDL Picture of the Week
September 11, 2017
Ripe Rot Confirmed in Indiana Vineyards
Bruce Bordelon, Professor,
Department of Horticulture and Landscape Architecture, Purdue University
There are a number of common grape pathogens that can cause
fruit rots each year in the region. Black rot and Phomopsis cane and leaf spot
are by far the most common fruit pathogens. We also see Botrytis gray mold on
some cultivars in cooler regions and years. Another common rot is Sour rot, but
it is actually caused by yeasts and bacteria, not filamentous fungi and is
spread by fruit flies. It occurs most often when heavy rains near harvest cause
berry splitting.
In the past few years we’ve had a troubling rot on a new
cultivar to the region, Marquette (Fig 1). This early ripening red has
excellent wine quality and is one of the new “super cold hardy” cultivars from
Minnesota. That makes it a great choice for northern Indiana vineyards.
However, Marquette is not without it’s disease problems. While only moderately
susceptible to black rot, it is highly susceptible to Phomopsis cane and leaf
spot. Extra measures taken to manage Phomopsis have not been successful at
stopping the fruit rot on Marquette. So we investigated a bit further to
discover that, in fact, a relatively unknown disease to our region causes the
rot. Ripe rot is caused by Colletotrichum
gloeosporioides and is common in the southern Atlantic wine growing
regions, but not the Midwest. It may be that the disease is spreading more
westward, or more likely, that Marquette is just particularly susceptible, as
we don’t tend to find this rot on other cultivars. Marquette ripens about 2
weeks earlier than most cultivars so it’s possible that higher temperatures
during ripening are partially to blame.
One concern with ripe rot is that it can infect fruit during
the ripening period. The other fruit rots we commonly deal with infect much
earlier in the season, so growers quit applying fungicides after mid-summer as
risk for infection is greatly diminished. The lack of late-season fungicide applications
may be why we are seeing ripe rot show up on susceptible cultivars. We will
have to change our recommendations to reflect this new threat. Captan, Ziram
and strobilurin fungicides have good activity against ripe rot.
The primary symptom of ripe rot is rotting of ripe fruit. Symptoms
are not common on leaves, shoots, or cluster stems (unlike Phomopsis that also
causes a rot of ripe fruit) (Fig 2,3). Affected berries develop circular,
reddish brown spots on their skins and the spots subsequently enlarge to
include entire berries. A key characteristic is that infected berries become
covered with salmon-colored spore masses of conidia as they decay (Fig 4). Eventually
diseased berries shrivel and resemble the terminal stages of several other rot
diseases (black rot, Phomopsis). Presence of the characteristic salmon-colored
conidia from acervuli is diagnostic for ripe rot.