What is Mildew?
Mildew is a fungal disease that causes the growth of fungal hyphae in the plant tissues, wood, leather, paper etc. Its relatively similar to powdery mildew but this fungal infection penetrates into the tissues of the leaves, stems and fruits and not only remains in the surface as it does the first one. As being an endoparasit, you won’t see its formation until the damages produced in the plant are considerable, these damages are usually caused in the upper parts of the plants. It’s part of the family of the Peronosporaceae, which includes 7 genres and approximately 600 species.
In agriculture it often damages crops of potato, grapevine, tobacco and of the cucurbitaceous family, without forgetting of course the cannabis crops. This pathogen is quite specialized because normally every species attack a specific – and relatively small number – of plants.
One of the most known species is Plasmopara viticola or grapewine downy mildew, which was introduced in Europe in 1878 by the French when they imported strains of American vine resistant to phylloxera, but hosts of this fungus.
Procreation of mildew in cannabis crops
Like so many other species of fungus, Mildew need particular climatic conditions for its reproduction, even if it’s able to remain latent for a long period of time before becoming active and attack a crop. The main source of infection often tends to be plant debris where the disease can stay to be disseminated later by external agents such as the wind, animals or ourselves. So as always we advise that maintaining our garden or crop clean of vegetable matter in decomposition will help to have healthier plants, especially in greenhouses and on growing spaces of marijuana with poor ventilation.
The fungus becomes active when the humidity rises and temperatures range between 10 and 25ºC. During the winter it tends to remain dormant waiting for optimal development conditions, this usually happens during the spring. When the temperature rises and the humidity is high (irrigation by spraying, rain, dew) the fungus will begin to develop and reproduce. The grapewine downy mildew (Plasmopara viticola) overwinters as a structure called oospores (if the winter is mild, it will do so as a mycelium), germinating when the temperature rises to 12ºC forming a sporangium that will be scattered by the wind and which only will need a bit of water (rain) to germinate. It will remain dormant below 10ºC and above 35ºC.
Crops that are dirty with plant debris in decomposition, with presence of water in liquid form, temperatures between 10 and 25ºC, crops poorly ventilated or those with little sunlight, etc. are, therefore, vulnerable to be attacked by this fungus. To abundant irrigation, fog, dew, and rains followed by warm days will greatly favour the germination of the spores. On greenhouse and indoor crops it’s especially advisable to check the ambient humidity and ventilation of the growing area.
Symptoms of mildew and the damages it causes on cannabis plants
As we have said, the mildew affects mainly the aerial parts of plants and fruits, stems and especially leaves. The first symptoms are whitish/yellow and slightly translucent spots in the bundle of the leaves (commonly called “oilspots”); if the humidity is high, it will form a white down on the underside of them easy to confuse with powdery mildew, although in the case of mildew if you graze this kind of downy dust we will find the oilspots.
If the infection propagates, the affected parts dry off and become necrotic, causing the premature fall of the leaves (defoliation) and affecting so the yield and final quality of the plant. As Autumn arrives, the spots delimited by the nerves of the leaves, develop mosaic tonalities (of yellow to brown) and if the moisture level remains high its possible that it creates white down.
Mildew also can affect the stems of the plants, forming a spot of brown color that sometimes can go so far as to turn around the stem. The infected area withers and can die in case of severe infection.
Prevention and control of mildew in cannabis plants
If your garden or crop is free of mildew, you should follow a series of preventive measures to avoid appearance. Keeping them clean of plant remains, to ensure a proper amount of sunshine and ventilation, don’t watering them unless it’s necessary, avoid high levels of humidity and a too high growing density, doing pruning to avoid a too leafy plant, etc. are some of the things you can do to prevent mildew.
As preventive treatments, you can opt for sprays with chamomile broth (50g of flowers per liter of water then dissolve at the rate of 9 liters of water per liter of broth), garlic (infusion of 50 g of garlic cloves per liter of water then dissolve 1 liter of ready broth in 4litres of water and spray it on the plant) you can also use horsetail or propolis. Other frequently used preventive treatments are Bordeaux mixture, copper oxychloride and Dithiocarbamates.
If, on the contrary, you detect symptoms of mildew in your crop you have to act quickly so to avoid to be ruined. Clarify the area of plant debris, in the case of poor aeration you can prune and remove the affected leaves. There are fungicide products that act by contact (penetrating) or with systemic action to combat this fungus. Those that act by contact (Climoxanilo, chlorothalonil, Imazalil) normally are used after 24 hours of the onset of favourable conditions for an attack, such as rain, while the systemically ones (metalaxyl, Fosetil-Al, Triadimenol, Tebuconazole, myclobutanyl and Bitertanol) are used after a maximum of 72 hours.
As always, we advise you to read carefully the instructions of use of these products and to follow in detail the for its use required security measures It’s preferable to use organic products respectful with the environment, both for your health as well as that of those around you and remember that it’s always better to prevent fungal infections than to cure them!