Mouton Noir

Sadly, vines can contract diseases. Here are the major must we need to watch out for and strategies to handle them.

Downy Mildew (Plasmopara viticola)

A fungus-like (oomycete) organism that causes yellowish patches on the leaves and can cause losses in yield and fruit quality.

• Conditions: Warm, wet, humid conditions.
• Susceptibility: Most susceptible in the spring and early summer, when new growth is abundant and weather conditions are often wet.
• Identification:
  • Leaves:
    • Roughly circular yellowish discolourations, called “oil spots”
    • White fluffy growth primarily on the lower leaf surface
    • As lesions age, they turn brown from the centre outward
    • Severely infected leaves may drop
  • Shoots:
    • Infected shoot tips curl (“shepherd’s crook”)
    • Covered with white fluffy sporulation
  • Rachis:
    • Severe infections will cause the rachis and cluster to twist like a corkscrew
    • Entire surface can be covered with sporulation
  • Berries:
    • White fluffy sporulation when shot-size
    • May shrivel and drop off
    • Berries of red cultivars infected between 3 and 5 mm in diameter size will turn colour prematurely
    • Those of white cultivars acquire a mottled appearance
    • Stay hard when healthy berries start to soften at veraison
• Conventional treatment: Application of fungicides.
• Organic treatment: Use of copper-based sprays, improving air circulation in the canopy and promoting biodiversity.

Note: Why copper? When applied correctly, a protective barrier of the copper compound coats the plant tissue. The copper compound releases copper ions in the presence of moisture which are passively taken up by the fungal spore of the downy mildew pathogen to the point that they stop germination and infection.

Powdery Mildew (Erysiphe necator)

A fungal disease that can cause stunted growth and reduce yield and fruit quality.

• Conditions: Warm and dry conditions, high humidity, poor air circulation.
• Susceptibility: Can strike at any time, but it's most prevalent in mid to late summer.
• Identification:
  • Leaves:
    • Frequently first found on the undersides of leaves
    • Lesions become apparent on the upper sides of leaves as well
    • Increase in size and number if the disease is left unchecked
    • On rapidly growing leaves, infections on the underside may cause the leaves to appear puckered on top
    • Severely infected leaves may become brittle and drop off
    • Starting as early as late July, very small orange then brown and eventually black spherical structures (cleistothecia) develop on the upper and lower surfaces of infected leaves
  • Shoots:
    • Initially greyish-white, develop into brown irregular blotches Up to a few cm
    • Indistinct margins and remain visible after shoot hardening
  • Rachis:
    • Severe infections make the rachis brittle
    • Can result in clusters being dropped, especially if mechanical harvesting is done
    • Symptoms on the rachis are similar to those on shoots
  • Berries:
    • Become covered in conidia
    • An initial floury appearance that later becomes dark and grey
    • Dry out and may drop off
    • Later infections (3-4 weeks post bloom) will have superficial greyish scarring but not a lot of mycelial growth or sporulation
    • On all tissues, powdery mildew looks like a greyish-white powder.
• Conventional treatment: Application of sulfur and synthetic fungicides.
• Organic treatment: Use of sulfur-based sprays and milk, increasing canopy air circulation, and selection of resistant varieties.

Note: Why milk? Scientists are not exactly sure how milk sprays work, but most think proteins in the milk interact with sun to create a brief antiseptic effect.

Botrytis Bunch Rot (Botrytis cinerea)

A fungal disease that can lead to significant loss of crop and reduction in fruit quality.

• Conditions: Cool, wet environments, especially during or after veraison.
• Susceptibility: Most susceptible from veraison to harvest, especially if the weather is persistently wet.
• Identification:
  • Leaves:
    • Infection begins as a dull green spot
    • Typically including the edge of the leaf blade, eventually becoming necrotic
    • When incubated under high humidity, produce greyish-tan spores
  • Rachis:
    • Infected areas dry out, causing berries below the affected area to shrivel
  • Berries:
    • White cultivars become brown and shriveled
    • Red cultivars become reddish-brown
    • Covered with greyish-tan conidia frequently first seen in tufts or along splits in the berries
    • Skin is easily removed from the flesh (“slip skin”)
    • If dry weather follows infection, infected berries will shrivel, and the infection will not progress (Noble rot)
• Conventional treatment: Use of synthetic fungicides and careful canopy management.
• Organic treatment: Canopy management, proper site selection, and the use of resistant grape varieties can help. Biological control agents and copper or sulfur sprays are also options.

The defense system of a vine

Vines, like other plants, use a sophisticated system of defences to ward off pests and diseases. Their “immune system” is based on recognising and responding to molecular signals from attacking organisms, combined with physical and chemical defences.

The defence system of a vine consists of two tiers of immunity: Pathogen-Associated Molecular Pattern-Triggered Immunity (PTI): The vine's initial response is to common pathogenic features, known as Pathogen-Associated Molecular Patterns (PAMPs). These are recognized by Pattern Recognition Receptors (PRRs) in the plant cells, triggering defense responses like strengthening of the cell wall, production of antimicrobial substances, and programmed cell death to inhibit pathogen growth.

Effector-triggered immunity (ETI): Some pathogens can evade the vine's initial defenses and infect the plant, releasing specific molecules called effectors into the plant cells to help the pathogen suppress the plant's immune response and colonize the plant. If the plant has resistance genes that recognize these effectors, it can initiate a more specific and strong defense response. This often includes localized cell death (hypersensitive response) to prevent the pathogen from spreading further.

Both these levels work together, and a vine's immune response will involve several physical, chemical, and cellular changes. It's also worth noting that the vine's ability to protect itself against diseases and pests is significantly influenced by environmental conditions and its overall health.

Systemic Acquired Resistance (SAR) is another aspect of a vine's defense mechanism, where an initial localized infection leads to an increased resistance throughout the plant. This is similar to immune memory in animals, where the immune system “remembers” previous infections and responds more effectively to subsequent encounters with the same pathogen.

To enhance a vine's natural defence system, it's crucial to maintain the overall health of the vine through adequate nutrition, water, and sunlight. Proper pruning practices and pest control also play a critical role. In some instances, the use of biological control agents (beneficial insects or microbes) or organic compounds that stimulate the plant's own defences can be helpful. Disease-resistant grape varieties are also a crucial strategy for managing diseases in vineyards.

However, our understanding of plant immunity is still growing, and future research will continue to shed light on how we can better protect our crops from pests and diseases.