Few things strike more fear into the hearts of farmers than the appearance of fuzzy or powdery growth on their plants. Hydroponic growers face the additional horror of finding roots engulfed by an unspeakably gross slime or rot away before their eyes. Ah, fungi and their relatives fungal protists… what a fascinating nightmare.
Molds and Mildews are Fungi
What are commonly called molds and mildews are usually members of kingdom Fungi. Fungi are eukaryotic (contain a nucleus), heterotrophic (obtain food from their environment) absorbers (they release enzymes into their environment to break down organic matter that they then reabsorb) that never contain chlorophyll (this separates them cleanly from all plants and plant-like organisms) and reproduce either sexually or asexually by budding or by the production of spores.
(A note on spores: spores are not like seeds. Seeds contain a complete multicellular plant embryo, while a spore is a single cell that is specially designed for dispersal and survival. Some spores die relatively rapidly if conditions are right, but other spores are more survivable and can remain dormant for quite some time until conditions are right for germination.)
This puts them in alliance with other eukaryotic organisms, such as plants and animals. Although the disease symptoms fungi exhibit can be similar to those that accompany bacterial infections, fungi are nothing like bacteria (since they are eukaryotes, fungi possess a much greater complexity of cellular organization and structure than bacteria).
Fungi are a diverse lot. They range from single celled yeasts like Saccharomyces cerevisiae—used in bread and beer making—to the colorful and majestic mushrooms like Amanita muscaria, which rapidly appears after a good rain. However, mycologists do not always agree on exactly how they should be classified.
Like plants, fungi have historically been classified on the appearance of their reproductive structures; however, not all fungi produce clearly observable reproductive structures. To make it even more difficult, some fungi produce different kinds of reproductive structures depending on the environment, or whether or not they are able to undergo sexual—as opposed to asexual—reproduction.
What we generally refer to as molds are fungal colonies that begin as a white fuzzy growth on some substrate, such as a piece of fruit or other bit of decaying organic material. However, the term mold is rather a catch-all, as it simply describes a fungal colony that is either producing no spores at all or eventually produces spores by asexual means only.
Asexual spores are generated by the standard eukaryotic cell division process called mitosis. With the exception of variations produced by random mutation, all mold spores are genetically alike. This is not a good strategy for adapting to new environments, but it is an excellent short-term survival strategy. Mold spores are produced by the millions and float about easily in the slighted movement of air, which is one reason why molds can be hard to get rid of.
Spores tend to arise near the center of the colony before radiating outward to cover the expanding mycelium. The mycelium is composed of a mass of fungal cells called hyphae, which are extending thread-like tubular cells. Mold spores are often quite colorful. Brown, black, green, blue, yellow, purple and shades of orange and red are commonly seen, and they provide an initial clue as to the species.
Fortunately for growers, most molds are saprophytes that gain their nutrients from already dead organic matter. However, some molds are decidedly plant parasites, either by nature or by opportunity. Plant diseases caused by fungi include rusts, smuts and various forms of rot.
Mildew...now there is an interesting term. It means different things at different times. Mildew is sometimes the name given to a foul-smelling and seemingly non-obvious fungus growing in a damp place where it is not supposed to. On the other hand, some fungal species have been given the specific names, such as powdery mildew or downy mildew.
A powdery mildew is usually a member of the fungal order Erysiphales, and the ones of specific interest to hydroponic growers belong to the genera Podosphaera and Erysiphe. Powdery mildews appear as white powdery spots, usually on the older lower leaves first, but can spread to cover the entire plant if unchecked.
The causative agents prefer moderate to high humidity and warm temperatures. The severity of the disease depends on many factors, including the variety of the host plant, the age and condition of the plant and the weather conditions during the growing season. Powdery mildews are a bit fussy, however, and a particular species will only infect a few types of plants.
Protecting against and treating fungal infections of plants
There are thousands of different plant diseases caused by fungi, including rusts, smuts, blights, damping off disease and rots. Fungal infections are particularly troublesome because fungal cells are in many ways similar to those of other eukaryotes (such as plants), which makes it difficult to find chemicals that will attack fungus, yet be harmless to plants and people. There are some treatments available for particular fungal diseases, although these treatments might be only partially effective.
Fortunately, most plants of commerce are available in disease resistant varieties. However, just because a plant is resistant to fungal infection, does not mean it is completely immune. Overcrowding, poor air circulation and unnecessarily high humidity will doom even the hardiest of varieties. And, even with the best of practices, sometimes fungal pathogens still appear. Like bacteria and viruses, fungi can find their way into a host plant through cuts or breaks in the plant epidermis (hence why it’s important to treat plants gently). Also, some fungi produce special penetrating hyphae that are capable of forcing their way into plant tissues or growing in through stomata—powdery mildew is one example.
Heirloom cultivars also pose a problem for they require nothing short of close inspection and rigid adherence to procedures for proper environmental conditions.
Treatment options for fungal diseases include pruning diseased leaves and stems or the complete destruction of diseased plants—better to intentionally destroy half a crop that to lose all of it to an unchecked infection. Diseased plants should be disposed of properly and not used in compost for future application. Anti-fungal treatment options that might be acceptable to organic and natural growers include compounds of copper and sulfur, or hydrogen peroxide.
Other methods that are based on plant oils—for example, neem oil, bicarbonates and lipopeptides (a combination of a fatty molecule and a protein fragment)—are under investigation. Some say they work and other say they don’t. If pruning, removal, or organic-chemical methods fail, commercial fungicides do work; although, not everyone will consider this approach acceptable.
With respect to the powdery and downy mildews, avoid overhead watering and application of nitrogen fertilizer late in the growing cycle. Mildews thrive on healthy turgid tissues such as those encouraged by application of nitrogen. A grower can also take proactive steps to protect a crop from attack by molds and mildews. Good air circulation, elimination of overcrowding, appropriate watering practices and vigilance are the key to consistently healthy and disease-free plants.
One of the most dreaded afflictions of plants in a hydroponics operation is pythium rot. Although pythium does appear to be rather fungus-like, it is actually a fungal protist. Close attention to hygiene, a solid program of system maintenance and cleaning, and the use of an ultraviolet sterilization lamp in recirculating nutrient systems can help prevent the appearance of pythium. Also, maintaining an adequate oxygen supply and a temperature between 68 and 72°F in the root zone makes it difficult for pythium to gain a foothold.