“What’s the difference between powdery mildew and downey mildew and what is used to treat them?” “What is this white powdery substance infesting my tomatoes? Pls help” are questions frequently asked by most first-time greenhouse farmers.
Let’s figure out this puzzle together..
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 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
Powdery Mildew Epidemiology
The severity depends on many factors:
Variety of the host plant – some are tolerant others not.
Age and condition of the plant – young and succulent crap more susceptible.
Severe in warm, dry climates. This fungus does not need the presence of water on the leaf surface for infection to occur.
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).
Crowded crop. Poor air circulation,damp,and shaded areas.
Powdery Mildew 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
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
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..
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 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.
Downey Mildew 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
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
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