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Botrytis, Powdery Mildew, and Fungicide Resistance

Botrytis, Powdery Mildew, and Fungicide Resistance

Botrytis Bunch Rot
In the North Coast, vineyard location determines if botrytis bunch rot will occur even in “normal” years. In other parts of the region, this disease usually only becomes a problem near the end of the season in varieties with tight clusters, especially if it rains near harvest. In 2011, late season bunch rot was severe in more vineyards across the county because of mild temperatures and rainfall during bloom and ripening.

A botrytis management spray program used by some growers in “always gets botrytis” growing areas may include four applications: bloom, (just prior to) bunch close, veraison and sometime preharvest. It is difficult to justify the costs of early sprays that may not be necessary if there is no in-season rainfall – which is a classic dilemma. In 2010 and 2011, applying botrytis fungicides at bloom and berry touch would have reduced disease severity during ripening UP TO A POINT.

Post veraison botrytis bunch rot requires two factors – mild temperatures and “free moisture”.  At temperatures of 65 to 75 °F (18 to 24 °C), only 2 hours of free water are needed for Botrytis cinerea spores to germinate and infect green tissue (1). Free moisture includes rain, dew and fog – which are common events.

Botrytis Disease Cycle
Botrytis cinerea conidia are ubiquitous in a vineyard. During the bloom period, flower parts and individual flowers that develop into berries, can become infected. Calyptra, stamens, and unfertilized flowers are infected under moist conditions during bloom.  Any time after bloom, when wet conditions occur, this floral debris will produce spores that can infect the rachis and other cluster parts.  During bloom, “latent” infections of individual flowers primarily occur in the very small gap between the ovary and calyx created when the calyptra (cap) pulled away (2). Spores are “washed” down to the gap with moisture, thus the receptacle area is where the fungus grows. [Photos 1 and 2.]

1. Gap between ovary and calyx, where the stamens and calyptra attach to the receptacle, is a common infection site. (Photo by UCIPM)
1. Gap between ovary and calyx, where the stamens and calyptra attach to the receptacle, is a common infection site. (Photo by UCIPM)

2. Grape flower after cap fall. (Photo by UCIPM)
2. Grape flower after cap fall. (Photo by UCIPM)

Bloom time flower infections remain latent. They may never cause disease prior to harvest.  What makes latent infections active after verasion is not well understood. Disease onset initially involves the internal tissues of the berry. The berry turns dark (easy to see in a white variety), the pulp softens and the “skin” easily splits or slips off the pulp when touched (“slip skin”). [Photo 3.]

3. Disease onset near harvest in Chardonnay. Latent infections of berries at bloom become active.
3. Disease onset near harvest in Chardonnay. Latent infections of berries at bloom become active.

Fungal structures grow out of the pulp exposed by the split skin, producing spores. These diseased berries are the primary source of spores that spread to adjacent berries and, by wind, to other clusters. [Photo 4.]

4. B. cinerea spores produced after diseased berries split. Spores can germinate on intact berry surfaces causing new infections.
4. B. cinerea spores produced after diseased berries split. Spores can germinate on intact berry surfaces causing new infections.

Botrytis spores can germinate and infect intact, healthy-appearing berries; wounds are not required for infection. The moisture on the berry surface contains sugars that “leak” through intact skin supplying sufficient nutrients to allow spores to germinate and infect.  [Photo 5.]

5. Diseased berries from latent and new infections.
5. Diseased berries from latent and new infections.

A cluster that was partially or fully diseased with botrytis bunch rot last fall, if left on the vine will support the black overwintering structures (sclerotia) of B. cinerea. Conidia produced by these structures provide the inoculum for the following spring. [Photo 6.]

6. Cluster “mummy” with overwintering Botrytis fruiting structures.
6. Cluster “mummy” with overwintering Botrytis fruiting structures.

2011 Had Multiple Infection Periods
Under wet prebloom conditions, botrytis shoot blight will occur on green tissue - shoots, leaf blades and unopened flower clusters – which sets up bloom time infections weeks later. This occurred in 2011; over two-inches of rain fell in mid May and an inch in early June. It remained foggy and damp through the second week of June when bloom was either just beginning or well underway. Under those conditions, flower parts (stamens and calyptras) and individual flowers became infected as previously described.  During the wet periods prior to veraison, spores produced by floral debris infected cluster parts.

As berries ripened, they became susceptible to disease from latent infections established at bloom. Many diseased berries eventually split and spores were produced in the cracks (Photo 5) that spread to other berries. Depending on the location and fungicide program, bunch rot was evident in varying degrees prior to October 3rd when over an inch of rain fell in two days.

During and immediately following that rain event, disease severity jumped; most people who still had fruit on their vines needed to get it off fast, regardless of sugar maturity. Conditions were ideal for the pathogen which can generate spores in as few as 3 days from a new infection. For that to happen the following had to occur (and did occur) - a) free moisture was present; 2) relative humidity exceeded 92%; and 3) temperatures ranged from 59 to 82°F (15 to 28°C) (1).

Wet conditions at bloom, coupled with mild temperatures and several overcast days during the “summer” in 2011 contributed to the disease epidemic at harvest.  Applying a bloom spray months earlier would not have been sufficient. Growers who applied (and re-applied) fungicides as the grapes accumulated sugar had mixed results.  Varieties with tight clusters were severely diseased; varieties with looser clusters fared slightly better.

1989 Revisited
In 1989, bunch rot at harvest was also memorable. Bloom time temperatures were mild. Climate data from the UCIPM database indicates local weather stations received precipitation on May 23 ranging from 0.05 to 0.18 inches and again on June 14 (up to 0.25 inches). Beginning September 16, up to 3 inches fell over 3 days across most of the county. In the following 10 days, a lot of fruit was picked.

Managing Disease
Fungicide applications, canopy management practices and an effective powdery mildew program all have a role in controlling Botrytis. Yet another source of infection is from “diffuse powdery mildew infections” reported to be associated with severe Botrytis infections at harvest (3).  Also, mildew scars on developing berries will cause cracks that increase botrytis severity.

Fungicides applied prior to rainfall can protect tissue better than if applied later. In some areas, under 2011 conditions, it may not have been possible to have prevented crop loss due to disease. The site, canopy conditions and fungicide usage pattern in previous years – for both botrytis and mildew – all affect control.

Going forward, reduce next year’s Botrytis inoculum by getting the cluster mummies off vines; chop the prunings as soon as feasible before bud break this spring.  Focus on protecting clusters at bloom in 2012, and take the time this winter to evaluate what fungicides were applied in 2011 and determine if changes are in order. Managing botrytis and powdery mildew includes managing resistance risk to specific fungicides in your vineyard.

Fungicide Modes of Action and Resistance Risk
The FRAC Code List© (http://www.frac.info) presents fungicide active ingredients organized by groups based on the biochemical mode of action in the metabolism of plant pathogens. Related chemicals are put into groups (also referred to as “classes”) with the same mode of action.  If the FRAC Code assigned to a specific group is included on a fungicide product label, it may be written as a “Group Number”.

The resistance risk (resistance potential) of a fungal pathogen to the active ingredients of a fungicide within a given FRAC group is estimated as “low”, “medium” or “high” based on several indicators. A chemical with a designation of high risk is likely to lose the ability to control the target pathogen unless resistance management practices are used.

Given the cost of product development and registration, it is in the best interest of manufacturers to recommend a strategy that will extend the lifespan of their product which may include using competitors’ material(s).  It is in the best interest of growers to follow a risk management strategy for all foliar fungicides applied in the season; keep track of product FRAC group numbers and mix or rotate products in different groups as directed.  This is true for both medium and high risk materials (Table 1).

Table 1:  Fungicides registered in California for use on grapes in selected FRAC Groups with estimated resistance risk, December 2011.a


Information, other than trade name, is from the FRAC Code List©. (Note: the 2011 Frac Code List has been taken off-line. The 2017 Frac Code List is available here). Fungicides included in this table are only those in the same FRAC Groups as the premix products in Table 2. 
A fungicide may have more than one trade name. 
Relative resistance risk depends on several factors. Based on research in specific crops, UC plant pathologists also have resistance risk designations in the publication 2011 EFFICACY AND TIMING OF FUNGICIDES, BACTERICIDES, AND BIOLOGICALS for DECIDUOUS TREE FRUIT, NUT, STRAWBERRY, AND VINE CROPS (4) (http://www.ipm.ucdavis.edu/PDF/PMG/fungicideefficacytiming.pdf)

“Cross resistance” of a pathogen to two or more active ingredients within the same group often results in an assignment of high risk to that group. Products that are Quinone outside Inhibitors (QoI) – FRAC group 11 – are high risk. There are mildew and botrytis products in FRAC Group 11; however a few are premix materials which contain a second active ingredient in a different group (Table 2).

Table 2:  Premix fungicides registered in California for use on grapes, December 2011a.


Does not include downy mildew fungicides.
b Group numbers are assigned by the Fungicide Resistance Action Committee (see footnote in Table 1).

Most conventional products used to control powdery mildew and botrytis bunch rot diseases are considered to have medium or high risk potential. A risk designation for a specific fungicide can vary because it is affected by how the material is being used and under what conditions. Thus, the geographical area and the severity of the disease in that area are assessed in addition to application number and timing.

Resistance has been found in California for certain fungicides with a single-site mode of action, therefore “medium” resistance risk designations by the FRAC may be increased to “high” by UC researchers (4).  This is true for cyprodinil, the active ingredient in Vangard, which is also one of the active ingredients in the premix fungicides Inspire Super and Switch. A fungicide with a single-site mode of action targets a single site in a specific biochemical pathway of a target organism. Fungi are more likely to develop a resistance to such products than they are for multi-site mode of action products.

Resistance development in a product is affected by the disease targeted (what is the pathogen’s ability to mutate and become less affected by the product?); weather conditions (are conditions ideal for disease development?); chemical and cultural disease management practices in the field (are the products you choose more or less prone to resistance problems?).

Powdery mildew and Botrytis bunch rot are controlled with multiple fungicide applications and each treatment can be a “selection event.” Even under perfect circumstances, an application will not kill 100% of the fungal population in the vineyard; this allows survivors to reproduce more resistant individuals. If and when those individuals dominate the population depends on several factors such as:

  1. Relative number of spores produced per generation.
  2. Varietal susceptibility to disease. A vineyard block of a susceptible variety is more likely to have a greater population of the pathogen than in a block of a less susceptible variety. Susceptibility includes cluster architecture; for example, tight clusters are more likely to get bunch rot then loose clusters.
  3. Spore dispersal mechanisms. Powdery mildew and botrytis spores can be moved by wind thus resistant individuals can be spread.
  4. Weather. Under optimal disease conditions, the number of fungal generations (from spore-to-spore) increases.
  5. Your disease control program.

A disease control program that reduces resistance risk to the product(s) in the spray tank requires attention to the following:

  1. Canopy management. Selective removal of lateral shoots and leaves to increase incidence of diffuse light (thus increase surface temperature and decrease relative humidity) on clusters and blades has been shown to reduce both powdery mildew and botrytis disease incidence and severity (5,6). Trellis and training systems as well as vine vigor affects cluster exposure to light and spray materials.
  2. Sprayer calibration. It is important to apply no less than the minimum labeled rate per acre to reduce resistance risk. In some resistance management programs, the highest rate of material is recommended. (4)
  3. Spray coverage. The fungus will not be killed on leaf blades and clusters that didn’t get sprayed. Alternate row applications in blocks with large canopies will reduce coverage.
  4. Choice of fungicides targeting powdery mildew and botrytis:
    • Make no more than one application of fungicides in FRAC Groups 9, 11 and 17 before rotating to a fungicide in a different group (4).
    • For fungicides groups other than 9, 11 and 17, make no more than two consecutive applications before rotating to a product in a different group (4).
    • When considering using a premix material, use Tables 1 and 2 to compare FRAC Groups with those in single active ingredient materials.
    • Alternate or tank mix a medium or high risk fungicide that has a single-site mode of action with a low risk material that has variable or multi-site mode of action (sulfur, Serenade, Sonata, JMS Stylet Oil, etc.). If tank mixing, check for compatibility.
    • Begin the disease control program with one or more applications of product that has a multi-site mode of action (7).


  1. “Bunch Rots” in Grape Pest Management – revision in preparation. UC Agriculture and Natural Resources, publication 3343.
  2. Virut, O., M. Keller, V.G. Jaudzems and F.M. Cole. 2004. Botrytis cinerea infection of grape flowers: light and electron microscopical studies of infections sites. Phytopathology, 94:850-857.
  3. Gadoury, D.M., R.C. Seem, W.F. Wilcox, T. Henick-Kling, L. Conterno, A. Day and A. Ficke. 2007. Effects of diffuse colonization of grape berries by Uncinula necator on bunch rots, berry microflora, and juice and wine quality. Phytopathology 97: 1356-1365.
  4. Adaskaveg, J., D. Gubler, T. Michailides and B. Holz. 2011 EFFICACY AND TIMING OF FUNGICIDES, BACTERICIDES, AND BIOLOGICALS for DECIDUOUS TREE FRUIT, NUT, STRAWBERRY, AND VINE CROPS http://www.ipm.ucdavis.edu/PDF/PMG/fungicideefficacytiming.pdf
  5. Carroll, J.E. and W.F. Wilcox. 2003. Effects of humidity on the development of Grapevine Powdery Mildew. Phytopathology 93: 1137-1144.
  6. English, J.T., C.S. Thomas, J.J. Marois, W.D. Gubler. 1989. Microclimates of grapevine canopies associated with leaf removal and control of Botrytis Bunch Rot. 79:395-401.
  7. Adaskaveg, J.E., H. Förster and D.Thompson. February 2008. “Resistance Management – a Necessity in Fungicide Usage.”Central Valley Post Harvest Newsletter – Issue 1: February 2008. http://postharvest.ucdavis.edu/libraries/publications/?ds=234&reportnumber=204&catcol=1809&categorysearch=Peach






To simplify information, trade names of products have been used. No endorsement of named products is intended, nor is criticism implied of similar products that are not mentioned.