Downy mildew is a common and destructive plant disease that affects a wide range of crops, including grapes, basil, lettuce, and impatiens. Its visual symptoms often appear similar to those caused by true fungi, leading to historical confusion regarding its classification. The organism responsible for downy mildew is definitively not a fungus, despite its name and filamentous growth pattern. This organism belongs to a distinct group of microorganisms known as Oomycetes, commonly referred to as water molds. Understanding this biological difference is important for effective disease management, as treatments designed for true fungi often fail to control Oomycete infections.
Oomycetes: The True Identity
Oomycetes were once mistakenly grouped with fungi due to their filamentous structures and their method of absorbing nutrients from a host. Modern biological classification places them in the Kingdom Chromista, or the clade Stramenopiles, alongside organisms such as brown algae and diatoms. The most telling difference lies in the composition of their cell walls, which are constructed primarily of cellulose and glucans. True fungi, by contrast, possess cell walls made of chitin.
Another major distinction is the ploidy of their vegetative stage. The Oomycete thallus is predominantly diploid (containing two sets of chromosomes), whereas the equivalent stage in most true fungi is haploid (containing one set of chromosomes). Oomycetes also produce motile, asexual spores called zoospores, which require water to swim and disperse.
Distinguishing Downy Mildew from Fungi
The common name “mildew” is the primary source of confusion, linking downy mildew to true fungal diseases like powdery mildew. Both types of mildew cause visible, mold-like growth on plant leaves and are significant agricultural pests.
The practical difference for a grower often comes down to the location of the growth and the environmental conditions required for infection. Downy mildew typically produces fuzzy, grayish-blue, or purplish growth almost exclusively on the underside of the leaf, often opposite a yellow spot on the upper surface. Powdery mildew, a true fungus, appears as a dry, white, powdery coating on the top surface of the leaves.
This distinction is driven by their biological needs, as Oomycetes require high humidity and a layer of free water on the leaf surface for their motile zoospores to initiate an infection. Downy mildew outbreaks are favored by cool temperatures (58°F to 78°F) combined with relative humidity above 85 percent. Powdery mildew, conversely, is often favored by high humidity but thrives in dry, warm climates where its spores are easily carried by air currents, and it does not require free water for infection.
How Downy Mildew Damages Plants
Downy mildew is classified as an obligate biotrophic parasite. The infection process begins when motile zoospores, or sometimes the sporangia themselves, land on a wet leaf surface and either swim or germinate to penetrate the plant tissue. Once inside, the organism extends its hyphae into the leaf cells to absorb nutrients.
The first visible symptom is usually the appearance of angular, yellow spots on the upper side of the leaves, called chlorotic lesions. These spots are angular because the infection is contained by the main veins of the leaf, preventing its spread into the surrounding tissue. As the disease progresses, the corresponding lower leaf surface develops the fluffy, downy growth, which is actually a forest of sporangiophores emerging through the leaf stomata.
These sporangiophores are the asexual reproductive structures that release new spores, spreading the disease to nearby plants. Severe infection leads to the rapid browning, wilting, and premature death of the leaves, a process called blight. Since photosynthesis is drastically reduced, the plant becomes weakened, resulting in poor yields, stunted growth, and the eventual collapse of the plant canopy.
Principles of Control and Management
Controlling downy mildew requires a management strategy that specifically addresses the organism’s unique biological need for moisture and its distinct cellular structure. Cultural controls are the first line of defense and focus on eliminating the film of water necessary for spore germination.
- Ensure ample space between plants and practice pruning to improve air circulation, which encourages leaves to dry quickly.
- Use drip irrigation or water plants early in the morning so that any moisture on the leaves evaporates before nightfall.
- Rapidly remove and destroy infected plant debris to reduce the amount of overwintering resting spores (oospores).
- Prioritize resistant cultivars where possible.
When chemical control is necessary, it is important to select products specifically effective against Oomycetes, often called oomicides. Standard fungicides that target ergosterol are completely ineffective against Oomycetes because they do not possess this compound. Effective active ingredients include phosphorous acid, mefenoxam, and oxathiapiprolin, which target the Oomycete’s unique biology. To prevent the rapid development of resistance, these specialty chemicals must be applied preventatively and rotated with other protectant chemicals, such as copper or mancozeb, which have different modes of action.