As promised,
this post will focus on Downy Mildew. The weather- here in the valley and province
in general, is warm, humid which unfortunately makes it very easy for Downy
Mildew to take hold.
This post
will detail why this disease is so destructive, the development cycle, and
possible preventative solutions. As mentioned in the previous post, it is much
easier to prevent a disease than to react to it once it is established. So
please, as always, go into veraison and harvest clean to maintain the best crop
quality possible, and into the dormant season with healthy vines.
Acknowledgement
to Wendy McFadden Smith of Brock and Katie Gold of Cornell University for their
research and publications on the topic. Please refer to these links for further
reading:
· The
lowdown on downy mildew in grapes – ONfruit
· Grape
Disease Control, Spring 2021 | Cornell Fruit Resources: Grapes
Additional
citations are at the end of this article.
Downy
Mildew (P. Viticola)-Developmental Conditions:
Downy mildew
is caused by a fungal-like pathogen and thrives in warm, humid regions. Out
of all seasonal diseases, mismanaged downy mildew is the only one that can
result in total vine loss.
The optimal temperature for P. viticola growth
is 25°C; however the infection is able to begin at
10°C. Rain is the main factor responsible for the epidemics.
The
growth of P. viticola may be slow to begin in northern regions, where sufficiently
high temperatures are not reached in the spring.
Damage:
Under the
right conditions, downy mildew infections can cause pre-mature defoliation,
which at best impedes critical post-veraison ripening, and at worst makes them
more susceptible to winter injury/kill.
Downy leads to organoleptic defects (changes in grape flavour profile), productivity reduction
and significant yield losses.
Please
Note:
Severe
downy mildew pressure in the prior season will likely result in an abundance of
primary inoculum in the following year’s early season.
Infection Sites:
Downy
mildew infects all green parts of the grapevine, including leaves, young stems
and grapes, but leaves were found to be the main source of spores due to surface size, and the lack of protection against invasion of the pathogen.
The disease is characterized by oily patches on the upper face of leaves
that develop a dull green or yellowish color while the underside of the leaf
exhibits a white growth.
Infection
of P. viticola causes: leaf discoloration, necrosis and
defoliation. Together this damage reduces nutrient composition, sugar
accumulation in berries, capacity for buds to overwinter and crop loss.
When
infected, young berries become brown and are covered by white powder resulting
from sporulation. As berries ripen, they become less susceptible to the
infection, but rachis infections can spread into older berries.
Figure
1.
Figure 2.
Fig 1: Multiple infections lead to a large number of lesions that may coalesce. (Courtesy G. Ash) https://www.apsnet.org/edcenter/disandpath/oomycete/pdlessons/Pages/DownyMildewGrape.aspx#:~:text=Although%20all%20green%20parts%20of,oilspots)%20(Figure%202).
Fig 2: After
suitably warm, humid nights, a white downy fungal growth will appear on the
underside of the leaves and other infected plant parts.
Figure 5. Figure 6.
Disease Cycle:
In the
spring, spores are released and infect vine tissue through rainfall.
Downy spores
can land in the soil, where they remain inactive until favorable conditions
occur. It will then transfer onto a host plant and start to grow. The life
cycle of this pathogen consists of primary and secondary infections. It
overwinters as oospores (resting sexual spores) in leaf litter, shoots, and
soil.
It takes about 7–10 days for symptoms of primary infection
to appear on the leaves, typically as yellow areas or “oil spots” that
later turn brown due to necrosis as the disease progresses.
Early
Season Infection:
Primary
infections begin when spores spread from leaf litter on the ground to young
leaves and clusters, beginning about 2-3 weeks before bloom. Suckers are
often the first infected because they’re closest to the ground. Berries are infected and
support profuse sporulation until 2 weeks post-bloom. Pedicel tissue remains
susceptible until 4 weeks post-bloom.
Unfortunately,
sanitation and dormant sprays have no effect on downy mildew, but early
season cultural management for other diseases provides an opportunity to scout
for these primary infections to see if your management to date has been
effective.
Early
season downy mildew management is essential for effective season-long
management.
If downy
mildew is mismanaged in the early season and becomes established, infections
will produce secondary inoculum season-long whenever conditions become
conducive, resulting in cascading late season epidemics.
Secondary
Inoculum
Secondary
inoculum release is triggered by warm, humid nights with rain shortly
thereafter. Without rain, most secondary inoculum will stay in place and die
the next day when exposed to bright sunlight. However, spores can survive and
remain infectious for several days between rainfalls if conditions remain
cloudy.
The requirements for sporulation and secondary infection are:
- Humidity of at least 98%
- Temperature of 13°C or above (optimum 20–25°C)
- At least 4 hours of darkness
- Leaves are wet for 2-3 hours pre-dawn.
All V.
vinifera clusters are highly susceptible from first shoot appearance through
approximately 4-5 weeks after bloom. Berries become resistant to direct downy mildew infection at
this time, but pedicels and foliage remain susceptible long after.
Multi-year
Infections:
Oospores can
survive for up to 2 years in the soil and can produce sporangia throughout the
growing season whenever conditions are suitable. Late summer infections are new
ones coming from oospores on the soil or in leaf litter rather than earlier
lesions on grape tissues. So not only can grapevines be infected by the
repeating cycle of sporangia on lesions, they can also be infected by new
sporangia from oospores.
It’s not
just the amount of rainfall that has an effect on sporangium release. The intensity of rain (millimeters per
hour) has a key role in splashing water drops from the ground to grape leaves;
increasing intensity results in an increased coverage by splashed drops, as
well as an increased number and size of the splashed drops. As rain intensity increases, mean raindrop
size increases and more splash drops are formed. Raindrop size influences both
the numbers of spores dispersed, and the distance dispersed. Large raindrops are more effective than small
ones because they carry more spores and can splash them farther.
Diagram of
Downy Mildew Development:
Prevention
and Control:
Practices
that encourage air circulation and quicker drying time can reduce disease pressure
but will not replace the need for chemical control. Plant debris should
be removed from the area around the plant and buried or burned.
All systemic fungicides for downy mildew management are prone to disease resistance development and should be used in rotation within a sound, integrated pest management program. Protectants used to control Phomopsis and/or black rot early in the season, will also provide good preventative control of downy mildew.
- Please consult the Perennia Pest Management Guide for effective products.
- Ensure complete coverage when spraying
- DO NOT skip rows.
- Tighten spray window if we receive excessive precipitation.
- Adhere to pre-harvest spray intervals.
I do hope
that this post well illustrates why early Downy Mildew control is essential in
vineyards.
Downy, Black
Rot… these diseases have found ways to overwinter and survive into the
following season. It is up to us to be vigilant in preventing it. Remember that
young vines- inter-plantings and new plantings require additional care as they are
getting established. They too require sprays- especially because they are trying
to establish themselves while dealing with environmental stressors.
Thank you all
for your time, and look out for the follow up post on pre-harvest updates this
week.
Cheers!
Katarina Vucic
Viticulture Specialist
Perennia Food and Agriculture
Email kvucic@perennia.ca
Office 902-678-7722
Cell 902-599-1390
Sources:
· Downy Mildew of Grape, by Ash G. Downy
mildew of grape (apsnet.org) Charles Sturt University, Wagga Wagga, New
South Wales, Australia.
· Koledenkova, Kseniia et al. Plasmopara viticola the Causal Agent
of Downy Mildew of Grapevine: From Its Taxonomy to Disease Management. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9130769/