Identification Guide to the Major Diseases of Grapes

Authors:
Odile Carisse, Agriculture and Agri-Food Canada
Réjean Bacon, Agriculture and Agri-Food Canada
Jacques Lasnier, Co-Lab Recherches et Développement
Wendy McFadden-Smith, McSmith Agricultural Research Services

Note: The purpose of this guide is to help growers and advisers properly recognize diseases of grapes in Eastern Canada.

Introduction

Vineyard

Rapid, accurate identification of diseases in the vineyard is key to preventing serious outbreaks and losses in yield and quality. However, the presence of a pathogen or disease does not necessarily mean that a treatment is required. The severity of diseases varies from year to year, depending primarily on weather conditions, the presence of inoculum (history of the disease) and the susceptibility of the vines. This means that a disease can be devastating one year and insignificant the next. The measures to be taken to prevent losses may therefore vary from season to season.

The purpose of this guide is to aid in the identification of grape diseases and in pest management decision-making. Accurate identification of pests is critical to an effective management program that provides optimal control while minimizing pesticide use. This guide describes the major diseases of grapes in Eastern Canada.

Legend

Favourable conditions for disease development

Risk for favourable weather conditions vs. Frequent periods with favourable weather conditions

Optimal temperature for disease development vs. Minimum and maximum temperature for disease development

Rainy with heavy showers and water accumulation on the ground

Scouting

When scouting a vineyard, bear in mind that there are several factors that can cause damage, including diseases, insects, herbicides, mammals, birds, cultural practices and abiotic factors (for example, weather). It is important not to draw premature conclusions regarding the cause of a problem and to keep in mind that more than one disease can be present at the same time. Ideally, a vineyard, regardless of whether or not it is in production, should be completely scouted, that is, all sections and all vines, at least once a week and ideally twice a week, from bud break to harvest. However this is not generally practical so special attention should be given to susceptible varieties or areas prone to disease, where the first symptoms of the disease generally appear. Well-targeted action taken early in the outbreak will generally result in improved disease control, often with less use of pesticides. Following harvest, it is important to check the condition of the vines before leaf drop (for example downy or powdery mildew sporulation) and after (for example anthracnose cankers on the cane) to assess the inoculum present on the leaves and cane at the end of the season. It is also easier to detect diseases that affect the wood of the vines after the leaves have dropped.

Examples of possible confusion during scouting

Some diseases have similar signs and symptoms.
A disease can have different signs and symptoms, sometimes depending on the grape variety.
Several diseases can be present at the same time.
Several factors, weather conditions, deficiencies, pesticides, can be present at the same time and can produce symptoms that are similar to those of diseases.

During scouting, particular attention must be paid to the following points

Clearly identify the block, susceptibility of grape variety (Appendix 1), and vine growth stage (Appendix 2).
Date of appearance of symptoms or damage, weather conditions before and during scouting.
Distribution of the damage within the vineyard, for example, localized, limited to certain grape varieties or to certain parts of the vineyard, such as a depression. It is important to cover all parts of the vineyard (relief, grape variety). Knowledge of the prevailing wind direction is important for spread orientation of wind dispersed diseases like powdery mildew.
The distribution of damage on the vine: all leaves, young or old leaves, upper side or underside of leaves, berries.
The description of the symptoms or damage: spot, scorch, rot, wilting. Check for steel wire or any other material that could injure plant tissues.

Example of scouting record sheet, downy mildew

Block: A1
Variety: Chancellor

Date: 17/08/2004
Growth Stage: berry touch
Weather: rain, high humidity, 22-24°C

Distribution of Symptoms or Damage:

In block: Located near the wooded area, in a depression, presence of water on the ground. Incidence: several vines, less than 1%
On vine: Primarily at the apex of the stems (Photo 1) White sporulation on the lower leaf surface, less than 25% of the vine affected

Description of Symptoms or Damage: Infected shoot tips curl and are covered with white sporulation (Photo 2) White sporulation can be removed with the fingers

(Consult Appendix 3 for blank scouting record sheets)

(1) Damage on vine, primarily at the apex of the stems. (2) Infected shoot tips curl and are covered with white sporulation.

Similar signs and symptoms of powdery mildew and downy mildew

Powdery mildew

Downy mildew

Description of this image follows. — Sporulating lesions - upper surface (30° angle) powdery appearance

Different signs and symptoms of powdery mildew

Diseases present at the same time

Damage from different sources

Hail - the falling hailstones create wounds on the berries — Hail

Powdery mildew - white spore giving the berries a floury appearance — Powdery mildew

Herbicide - Glyphosate: wilting of the leaves — Herbicide - Glyphosate

Frost injury on buds - leaves may turn brown and dry out, but older leaves may remain alive and display a pattern of flecking with sectors of yellow, white and green. — Frost injury on buds

Potassium deficiency - leaves may have a bronze color or some leaves may have dark spots or blotches — Potassium deficiency

Downy mildew: yellow areas visible on the upper surface of the leaves. As lesions mature, they expand rapidly and turn brown — Downy mildew

Management of grape diseases

Prevention

There are a number of factors that must be considered in selecting grape varieties. Planting varieties that are less susceptible to disease is a good way to prevent outbreaks. Orienting rows N-S and using topographical gradients will maximize air and soil drainage. Pruning debris may act as a reservoir for inoculum and should be removed and burned. Working the soil in the spring is a form of cultural control. Destroying and burying infected debris will reduce pathogen populations at the start of the season, therefore reducing the risk of disease.

Canopy management (pruning, shoot positioning, leaf removal) facilitates air circulation, which promotes drying of the leaves and increased penetration of fungicides. It is also recommended that vineyards be systematically inspected throughout the growing season to monitor the appearance and development of diseases. At harvest and at the end of the season, assess the presence of diseases like downy and powdery mildew or anthracnose on the leaves to estimate the level of inoculum that will be present the following spring.

Control

There are a number of fungicide application strategies. Where minimum risk is desired, fungicides can be applied in keeping with an established schedule rather than on the basis of observations made in the vineyard. This approach is not without its consequences on production costs and the environment. It is also possible to develop rational IPM programs for diseases of grapes. In such cases, the decision to spray is made after evaluating the consequences of this decision on production and the environment. In other words, the decision will be based on factors such as the weather conditions, the stage of development of the vine, the grape variety, parasitic pressure, etc.

Major grape diseases

Grapes are a crop that is susceptible to many diseases. However, the degree of susceptibility varies depending on the variety. When no pest management is carried out, damage can generally be severe. Downy mildew (a), powdery mildew (b), grey mold (c), anthracnose (d), black rot (e) and crown gall (f) are the major grape diseases in Eastern Canada. Anthracnose is not a problem in Ontario to date. Downy mildew, powdery mildew, grey mold, black rot and anthracnose are caused by fungi that attack the berries, reducing yield and quality. Crown gall is caused by a bacterium and can kill the plant.

The first step in an effective pest management program is correct identification of the disease. Since the severity of the damage and losses varies from year to year, the pest management program will vary depending on the weather conditions, grape variety and abundance of the pathogen.

White variety berries acquire a mottled appearance — Downy mildew

White spore giving the berries a floury appearance — Powdery mildew

Infected berries become covered with a greyish felt-like substance consisting of spores of the fungus — Grey mold

Deep spots, violet turning greyish in the centre, with a black edge. — Anthracnose

1. Berries become whitish then purple to black. 2. Berries will dry and become mummified but most of the time will stay on the rachis. 3. Berries will be covered by black pustules. — Black rot

The surface of the galls is irregular and the epidermis of the stem peels. — Crown gall

Downy mildew

Fungus: Plasmopara viticola
(Mildiou)

Symptoms

Leaves: Roughly circular yellowish discolorations, called oil spots (a). White down (sporulation of the fungus) (b), particularly on the lower leaf surface. The spots turn brown with time (c) and severely infected leaves may drop.

Shoots: Infected shoot tips curl ("shepherd's crook") (d) and a white down occurs on the stem (sporulation of the fungus) (e).

Inflorescences: The inflorescences are highly susceptible. During severe infections, they can turn yellow, brown, then dry out completely (f). They may also exhibit the cork-screwing observed in infected shoots.

Berries: White down (sporulation of the fungus) (g). Berries that exhibit sporulation when they are shot-size may shrivel and drop off. Infected berries of red varieties turn colour prematurely while those on white varieties acquire a mottled appearance. Infected berries tend to stay hard when the rest of the cluster starts to soften at veraison.

Downy Mildew Symptoms

Epidemiology

The causal fungus, Plasmopara viticola, is an obligate parasite that overwinters as oospores, or sexual spores, in dead leaves. The quantity of mature oospores in the spring is determined by the amount of precipitation the preceding fall.

In the spring, the minimum temperature for the development of the fungus is 11°C. At maturity, the oospores produce new spores (sporangia). Rain (presence of free water) then becomes the principal factor in the development of the disease. During heavy rainfall, the spores are dispersed to the leaves and fruit by water splash where they release another type of spore (zoospores, motile spores) which infect grape tissue.

Young grape tissue tends to be more susceptible to downy mildew than older tissue. Once leaves are fully expanded they tend to be less susceptible to infection. Fruit are susceptible only from bloom through 4 weeks post-bloom. However, since new succulent leaves are produced throughout the growing season, it is important to maintain good fungicide coverage for downy mildew.

Scouting

Grape varieties: Monitor susceptible varieties first.

When: From first leaf unfolded (stages 5-7), particularly during periods of rain, heavy dew and persistent fog at temperature above 11°C.

Where: Give priority to sections of the vineyard that remain wet: heavy soil, poor drainage, depression, abundant foliage. Downy mildew tends to develop on the shadiest side of the canopy first.

How: Monitor for the appearance of oil spots on the upper surface of the leaves. A white, downy sporulation that can be removed with the fingers can be seen on the lower leaf surface. Check for infected shoot tips, which bend in a "shepherd's hook", and become white with sporulation.

Early in the season, it is sometimes difficult to differentiate between downy mildew and normal "blotchiness" of young leaves. Suspected downy mildew lesions can be confirmed by placing leaves with symptoms in a plastic back with damp paper towel over night. Downy white sporulation will be evident on the underside of oilspots if it is downy mildew.

Weather conducive to disease development