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Downy mildew
Scientific name:
Albugo spp., Bremia spp., Peronospora spp., Pseudoperonospora spp.,
Order/Family:
Peronosporales: Peronosporaceae
Type:
disease (fungal)
Host plants:
Cabbage/Kale, Brassicas  Cucumber  Millet  Onion  Peas  Pumpkin  Soybean  Spinach  Watermelon  Zucchini/Courgette  Grapes, hops  
General Information on Disease and Damage
Geographical distribution
Geographical Distribution of Downy mildew in Africa (red marked)
Damage
Damage caused by downy mildews is usually associated with the sporulation of the fungus. Sporulation of Peronospora destructor can cause up to 55% reduction in the dry weight of onion leaves (Yarwood, 1941). The corresponding figures for Pseudoperonospora humuli on hops and Peronospora farinose on spinach were 17 and 48%, respectively.
Losses from downy mildews can be considerable. It is estimated that in 1962 downy mildew of tobacco (Perenospora tabacina) reduced yields in Europe by at least 100,000 metric tons (Peyrot, 1962).

Symptoms
Plants can be infected at any time during their growing period. Symptoms of downy mildew infection include small, pale yellow spots with indefinite borders on the upper leaf surface. Purplish discolouration of the upper leaf surface is seen on some hosts. A downy growth (sporangiophores) may be seen directly under the spots on the underside of the leaf or on fruits or stems early in the morning or when foliage is wet. Young leaves and cotyledons may drop off when yellow. Thus, the disease can cause severe damage to seedlings in the seedbed. Older leaves usually remain attached, and affected areas enlarge, turning brown and papery. When the disease is severe, whole leaves die.

Downy mildew on cabbage
© A.M. Varela, icipe
Affected plant stages
Seedling stage, vegetative growing stage, flowering stage and fruiting stage.

Affected plant parts
Leaves and whole plant.

Symptoms on affected plant part
Leaves: lesions; fungal growth.
Stems: fungal growth.
Flowers: fungal growth; flower abortion; flower drop.
Fruiting stage: fungal growth.
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Biology and Ecology of Downy Mildew
Several different fungi cause downy mildew disease on vegetables, fruits, ornamentals, forages, field crops and tobacco. These include Albugo spp. (crucifers), Bremia spp. (lettuce), Peronospora spp. (cabbage, tobacco, spinach, soybeans, alfalfa, onion, many ornamentals), Plasmopara spp. (grape and sunflower), Pseudoperonospora (cucurbits), Peronosclerospora (sorghum and maize), Sclerospora (grasses, millet), and Sclerophthora (maize, rice, wheat).

Downy mildew fungi are fairly host specific. The downy mildew fungus that infects one type of plant (e.g., rose) is not the same downy mildew fungus that infects another (e.g., grape). However, if you see downy mildew on one plant, then environmental conditions (i.e., cool, wet weather) are favourable for development of downy mildews on a wide range of plants.

Downy mildew of grape, spinach, and tobacco causes serious economic losses. It spreads rapidly through fields and is dependent on a wet, humid environment with cool or warm, but not hot, temperatures. A film of water is needed on plant tissue for spore germination and infection.

Conditions that favour development include:
  • Cool, moist weather conditions
  • Host weeds found in between the crops
  • Crop residues in the field
  • Poor plant aeration
  • Overcrowding (planting in high densities)

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Pest and Disease Management
Pest and disease Management: General illustration of the concept of infonet-biovision
This illustration shows the methods promoted on infonet-biovision. The methods shown at the top have a long-term effect, while methods shown at the bottom have a short-term effect. In organic farming systems, methods with a long-term effect are the basis of crop production and should be of preference. On the other hand methods with a short-term effect should be used in emergencies only. On infonet we do not promote synthetic pesticides.

Further below you find concrete preventive and curative methods against Downy mildew.
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Cultural practices

Prevention
1. Use resistant varieties where available
2. Use only certified diseased-free seeds for sowing. Transplant only healthy seedlings.
3. Ensure proper land preparation to make sure that your soil is well drained.
4. Provide adequate plant spacing to reduce the density of the canopy and minimise humidity.
5. Pruning of new growth also helps proper plant's aeration.
6. Remove infected plants and prune infected shoots.
7. Properly dispose of collected diseased-parts either by burning or burying them.
8. Avoid overhead watering. It lengthens the duration of leaf wetness and favours further development of the disease.
9. Plough-under all the plant debris after harvest.
10. Practice crop rotation.

Control options
Control should be emphasised in nurseries since downy mildew is particularly damaging in the seedbed.

1. Seedbeds should have well-drained soils and be sited away from hedges and windbreaks. The site should not have been under susceptible crops for at least the previous 2 years.
2. Seedlings should not be excessively watered.
3. Weeds should be eradicated in and near seedbeds and out in the production fields.
4. Crop residues should be removed from the field after harvest.
5. Avoid sprinkler irrigation.
6. Thin plants to reduce plant density and increase air movement.
7. Time irrigations so that they do not elongate leaf wetness.
8. Alter planting dates to avoid periods of high disease pressure.
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Biopesticides and physical methods

Copper

There are many copper compounds that are used as fungicides. The most common are derived from either copper hydroxide or copper oxychloride. These products are readily available in most third world countries and very reasonably priced. Copper products are still accepted in organic farming provided that the number of applications is strictly followed and a proper soil amendment is observed to prevent copper accumulation in the soil. For information on Copper click here.

Garlic bulb extract

For information on garlic bulb extract click here.
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Information Source Links
  • Dobson, H., Cooper, J., Manyangarirwa, W., Karuma, J. and Chiimba, W. (2002). Integrated Vegetable Pest Management. Natural Resources Institute (UK). ISBN: 0-85954-536-9.
  • ICIPE (2003).Varela, A.M., Seif, A.A. and Löhr, B. (2003). A Guide to IPM in Brassicas Production in Eastern and Southern Africa. ICIPE, Kenya. ISBN: 92 9064 148 7 www.icipe.org
  • OISAT: Organisation for Non-Chemical Pest Management in the Tropics www.oisat.org
  • Olis, J. and Hudelson, B. (2001). Downy mildew. BS thesis in Plant Pathology at the University of Wisconsin Madison. UW-Madison Plant Pathology. http://hort.uwex.edu
  • Peyrot, J. (1962). Tobacco blue mould in Europe. FAO Plant Protection Bulletin 10, 73-80
  • Wheeler, B.E.J. (1972). An Introduction to Plant Diseases. The English Language Book Society and John Wiley & Sons Ltd. ISBN: 0 471 03752 5
  • Yarwood, C. E. (1941). Sporulation injury associated with downy mildew infections. Phytopathology, 31, 741-748
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Downy mildew on cabbage
© A.M. Varela, icipe
Crop rotation
Crop rotation is the practice of growing different crops in succession on the same land.
Cucurbits
Cucurbitaceae is a plant family commonly known as melons, gourds or cucurbits and includes crops like cucumbers, squashes (including pumpkins), luffas, melons and watermelons. The family is predominantly distributed around the tropics, where those with edible fruits were amongst the earliest cultivated plants in both the Old and New Worlds.
Brassicaceae
Brassicaceae or Cruciferae, also known as the crucifers, the mustard family or cabbage family is a family of flowering plants. The name Brassicaceae is derived from the included genus Brassica. The family contains well-known species such as Brassica oleracea (cabbage, cauliflower...), Brassica rapa (turnip, Chinese cabbage...), Brassica napus (rapeseed...) and many more.
Brassicaceae
Brassicaceae or Cruciferae, also known as the crucifers, the mustard family or cabbage family is a family of flowering plants. The name Brassicaceae is derived from the included genus Brassica. The family contains well-known species such as Brassica oleracea (cabbage, cauliflower...), Brassica rapa (turnip, Chinese cabbage...), Brassica napus (rapeseed...) and many more.
Synthetic
Manufactured by chemical and industrial processes. May include products not found in nature, or simulation of products from natural sources (but not extracted from natural raw materials).
Fungi
Occurring worldwide, most fungi are largely invisible to the naked eye, living for the most part in soil, dead matter, and as symbionts of plants, animals, or other fungi. They perform an essential role in all ecosystems in decomposing organic matter and are indispensable in nutrient cycling and exchange. Some fungi become noticeable when fruiting, either as mushrooms or molds.

Fungi are responsible for a range of serious plant diseases such as blight, grey mould, bunts, powdery mildew, and downy mildew. Crops of all kinds often suffer heavy losses.

Fungal plant diseases are usually managed with applications of chemical fungicides or heavy metals. In some cases, conventional breeding has provided fungus resistantcultivars.

Besides combatting yield losses, preventing fungal infection keeps crops free of toxic compounds produced by some pathogenic fungi. These compounds, often referred to as mycotoxins, can affect affect the immune system and disrupt hormone balances. Some mycotoxins are carcinogenic.
Label
is any written, printed or graphic presentation that is present on a product, accompanies the product, or is displayed near the product.
IPM
A combination of chemical and biological control methods, based on the concept of economic tresholds. Pest management in organic farming uses many biological control methods developed as par of IPM.
Cotyledon
The first leaf of a germinating seed, also called seed-leaf.
organic
Refers to the farming system and products described in the IFOAM standard and not to 'organic chemistry'.
Fungi
Occurring worldwide, most fungi are largely invisible to the naked eye, living for the most part in soil, dead matter, and as symbionts of plants, animals, or other fungi. They perform an essential role in all ecosystems in decomposing organic matter and are indispensable in nutrient cycling and exchange. Some fungi become noticeable when fruiting, either as mushrooms or molds.

Fungi are responsible for a range of serious plant diseases such as blight, grey mould, bunts, powdery mildew, and downy mildew. Crops of all kinds often suffer heavy losses.

Fungal plant diseases are usually managed with applications of chemical fungicides or heavy metals. In some cases, conventional breeding has provided fungus resistantcultivars.

Besides combatting yield losses, preventing fungal infection keeps crops free of toxic compounds produced by some pathogenic fungi. These compounds, often referred to as mycotoxins, can affect affect the immune system and disrupt hormone balances. Some mycotoxins are carcinogenic.
Cruciferae
Cruciferae or Brassicaceae, also known as the crucifers, the mustard family or cabbage family is a family of flowering plants. The name Brassicaceae is derived from the included genus Brassica. The family contains well-known species such as Brassica oleracea (cabbage, cauliflower...), Brassica rapa (turnip, Chinese cabbage...), Brassica napus (rapeseed...) and many more.
sporangiophore
Stalk or branch bearing spore cases.
Symptom
Symptom - the plant's response to the disease causing organism, examples are; changes in plant color, death of infected tissues, and wilting. It is the external or internal physical characteristic of a disease as expressed by the host plant.