Probably the best-known plant disease that repeatedly appears in gardens is powdery mildew. This is a fungal disease that combines various pathogens, some of which have very similar symptoms. Due to the different symptoms, powdery mildew fungi can be divided into two categories: 'true powdery mildew' and 'downy mildew'. In this article, you will learn how to differentiate between the mildew groups, how to prevent them and what to do if your plants are already infested.
Powdery mildew is one of the most common plant diseases in the garden. As already mentioned, mildew can be caused by various fungal pathogens. Powdery mildew is just a collective term for fungal diseases from different genera. Some fungal genera can be super host-specific, while others can affect many different plants. However, it is only important to differentiate between "downy" and "powdery" mildew when it comes to control and remedial measures. The two groups of fungi have completely different requirements and can therefore be combated and prevented in different ways. You can find out How to Recognize Whether You Are Dealing with a Plant Disease or the Wrong Care/Location in this article.
Powdery Mildew | Downy Mildew | |
---|---|---|
Distribution | Fair-weather fungus: Spread mainly by wind or splashing water (through watering). Occurs mainly in summer in warm and dry weather. | Bad weather fungus: Spores spread through water on leaves. Occurs mainly in damp, cold weather in spring or autumn. |
Symptoms | whitish-mealy coating on the upper side of the leaf, can be wiped off | purple-white fungal plexus on underside of leaf, difficult to wipe off, yellowish-brown upper side of leaf |
Susceptible Plants | Trees are also often infested (apples, grapevines, gooseberries, roses) | Only isolated infestation of woody plants (roses, vines). The focus is more on herbaceous plants. |
Powdery mildew (Erysiphaceae) includes various genera from theAscomycota division. These include the economically relevant species such as grass powdery mildew (Blumeria graminis) or vine powdery mildew (Erysiphe necator or Oidium). Apples are attacked by Podosphaera leucotricha, while roses are infested by Sphaerotheca pannosa. This was just a brief insight into the diversity of powdery mildew fungi.
Powdery mildew does not need damp conditions to spread ("fair weather fungus"), as the spores do not necessarily need water to spread. Only the first spores in spring can get from the soil to the leaf surfaces through spray water. In most cases, however, wind plays an important role as a means of dispersal. This is why powdery mildew often occurs in summer in dry weather compared to downy mildew. In addition, strong temperature fluctuations and alternating damp and very dry conditions can weaken the plants and thus promote infestation. After an infestation, the spores can overwinter in the plant remains and re-infect plants in your bed the following year through wind or spray water. It is therefore best to dispose of the plant remains of infested specimens after harvesting (do not compost).
Downy mildew comprises several genera of fungi that belong to the group ofoomycetes. The various species form the order Peronosporales. Important representatives of the group are downy mildew on grapes (Plasmopara viticola) and lettuce (Bremia lactucae). Many species of downy mildew are host-specific and therefore strongly dependent on their respective host plant species.
Downy mildew is known as a "bad weather fungus" because it normally occurs in damp, cool weather. Therefore, infestation occurs mainly in spring or fall. Cool, rainy summers also provide good conditions for downy mildew. This is because the spores of the fungus use water as a means of spreading. If the leaves do not dry quickly enough, the spores can be transported in the water film and enter the stomata of the leaves. Overwintering in fallen leaves or crop residues. If in doubt, dispose of all plant residues (not in compost) so that re-infection is made more difficult the following year.
I hope I have been able to help you differentiate between, prevent and combat powdery mildew and downy mildew.
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Cover image: Jeff Kubina, Public domain, via Wikimedia Commons
Isabell studies agricultural sciences and loves to be surprised by nature and its complexity again and again. Herbs - whether gathered wild or in the garden - are her passion.
Learn MoreWhat are the main differences between powdery mildew and downy mildew?
Powdery mildew forms a wipeable, whitish coating mainly on the upper side of the leaf and occurs in dry weather. Downy mildew appears as a purplish-white fungal web on the underside of the leaves that is difficult to wipe off and occurs in damp weather.
Which plants are susceptible to powdery mildew?
Susceptible plants include cucumbers, cucurbits, carrots, peas, sage, strawberries, laurel, vines, apple trees, gooseberries, roses, delphiniums, asters, golden balm and hydrangeas.
Which plants are susceptible to downy mildew?
Downy mildew mainly affects radishes, radishes, lettuce, peas, lamb's lettuce, cabbage, spinach, onions, roses and grapevines.
Garlic tea, horsetail decoction or nettle slurry are suitable for combating mildew.