“What is this white powdery substance infesting my tomatoes? Pls help” is a question frequently asked by most first-time greenhouse farmers. Chances are, it’s powdery mildew. Another common question is “What’s the difference between powdery mildew and downey mildew and what is used to treat them?” Let’s figure out this puzzle together..

Powdery Mildew

Powdery mildew is a fungal disease that causes destruction of leaves, stems, and flowers. Most common and easily recognized fungal disease. Affects all kinds of plants – cereals, grasses, vegetables, ornamentals, weeds, shrubs, fruit trees, forest trees. Has fast life cycle and produce white powdery patches on leaf surface and flower buds. Reduce aesthetic and economic value of crops Host specific and survives wide climatic range.

Powdery mildew

Capsicum leaf infected by powdery Mildew. A visible characteristic of the disease is the spots or patches of white to grayish, talcum-powder-like growth

Symptoms

  • Characterized by spots or patches of white to grayish, talcum-powder-like growth.
  • Tiny, pinhead-sized, spherical fruiting structures that are first white, later yellow-brown and finally black, may be present singly or in a group.
  • Most commonly observed on the upper sides of the leaves but affects the bottom side as well, young stems, buds, flowers and young fruit

Epidemiology

The severity depends on many factors:

  • Crop factors:

  1. Variety of the host plant – some are tolerant others not.
  2. Age and condition of the plant – young and succulent crap more susceptible.
  • Environmental factors:

  1. Severe in warm, dry climates. This fungus does not need the presence of water on the leaf surface for infection to occur.
  2. High relative humidity required for spore germination. Infection increases as relative humidity rises to 90 percent, but it does not occur when leaf surfaces is wet (e,g., in a rain shower).
  3. Crowded crop. Poor air circulation,damp,and shaded areas.

Disease Cycle

  • All powdery mildew fungi require living plant tissue to grow.
  • Powdery mildew survives from one season to the next as vegetative strands, called cleistothecia, on the bark of branches and stems.
  • Fungi grow as thin layers of mycelium on the surface of the affected plant parts.
  • Spores, which can be seen with a hand lens, are part of the white, powdery appearance and are produced in chains on upper or lower leaf surfaces.
  • In contrast, downy mildew, produces visible powdery growth, has spores that grow on branched stalks and look like tiny trees.
  • Moderate temperatures (15′ to 26°C) and shady conditions generally are the most favor-able for powdery mildew development
  • Leaf temperatures above 32°C, some spores may be killed.

Challenges in Powdery Mildew Management – grower perspective

  • Effectiveness of products – many products but only a few can be relied on when pressure is high.
  • Fewer application methods – all chemicals work through foliar application (reduces crop working time)
  • Certification issues with growers having direct market access reducing the range of available products
  • Generation of effective spray programme
  • Obtaining good coverage – thick crop canopy, dilution issues

Control Challenges

  • Infection is through out the year but inter during dry weather.
  • Certification bodies and their restrictions
  • Misuse/misapplication of good products by growers
  • Competition for resources within farms may lead to introduction of fake substandard pesticides

Chemical Control Of Powdery Mildew

  • Curative and protectant pesticides are applied for PM management
  • Excellent control when used as preventive treatments rather than curative treatments.
  • Once the fungus has started to sporulate controlling additional symptoms is difficult
  • Preventive fungicides should be applied when weather is ideal for disease development
  • Curative fungicides – use at early onset of visible signs
  • Spray coverage – Check on rates, water volumes, speed of applicators and chemical agitation
  • Nozzles-select nozzles that give good coverage

Now let’s switch gears and explore Downey Mildew..

Downey Mildew

Downy mildew is a fungal disease that causes destruction of leaves, stems, and flowers. Downy mildew species are: Peronospora, Bremia, Plasmopara, and Basidiophora. Downy mildew is a serious problem in the ornamental industry.

Downey Mildew

Downey Mildew fungus on the undersides of a Basil leaf

Signs and symptoms

  • At infection the fruiting structures of the fungus emerge from the undersides of the leaves and create the greyish – colored, downy coating.
  • The downy mass of spores are difficult to see without a hand lens or microscope. These spores appear on the underside of leaf lesions.
  • Purplish red to dark brown, irregular spots on leaves.
  • Angular blotches, yellow, purple to brown, to a scorch like burn.
  • Reddened areas on sepals and stem.
  • Small spots or long purplish areas may form on canes and may kill twigs
  • Infection usually occurs on young plant parts but other parts are also affected.
  • Defoliation may occur in extreme cases.

Disease cycle 

  • The fungus produces spores only on living plants.
  • The fungus survives in plant debris as resting spores (oospores) over unfavourable periods or on infected plants.
  • Cuttings taken from infected stock will carry over the disease.
  • Take home message is to check plants purchases and budwood carefully for this disease.
  • The fungus thrives in cool, humid (85% relative humidity) conditions
  • Roses are unaffected by downy mildew when humidity Is less than 85%
  • The optimum temperature for downy mildew is 18°C
  • Spores form more readily in cool, humid conditions
  • Once conditions turn warm and dry, the disease is kept in check
  • In the presence of water from a recent rain or heavy fog, the spores will germinate within four hours
  • Below 11°C, the spores won’t germinate, and they are killed by exposure to 28°C temperatures for 24 hours
  • Dry winds and warm, clear days inhibit spore production
  • Unlike powdery mildew spores, which are spread by the wind, downy mildew is spread by splashing water

Control of Downey Mildew

Cultural control

Manage humidity – It is critical to keep relative humidity below 85% to decrease sporulation on infected plants and stop germination of spores on healthy plants. Venting and heating if done well can reduce incidences

Hygiene

  • Critical in reducing pathogen spores
  • Sanitation requirements for the disease are stringent
  • • Rake leaves and debris that might contain overwintering spores
  • Discard debris away from production area
  • Roguing out infected shoots, buds and branches that might harbor disease from season to season.

Start clean

  • Take cuttings from plants that have not suffered a previous attack of downy mildew
  • Only plant clean stocks

Chemical control

  • Many products give excellent control when used as preventive treatments rather than curative treatments
  • Once the fungus has started to sporulate, symptoms cannot be reversed
  • Preventive fungicides should be applied when weather is ideal for disease development

Factors to consider when choosing chemicals

  • Stage of disease development- protect vs curatives
  • Efficacy- key to decision making. Has bearing in long term costs and environmental load, resources allocation.
  • Translocation in plant – contact or systemic
  • Presence of other diseases e.g botrytis, powdery – reduce spray load by selecting broad spectrum fungicides
  • Residues and environmental contamination – select fungicides with minimal residues
  • Resistance management-rotate chemical according to MOA and FRAC codes

Adapted from Floriculture Directory 2017/2018 Edition