Fungus

Just like animals, plants can get sick. Any plant caretaker knows how quickly a seemingly healthy specimen can succumb to a mysterious malady in just a matter of days. It can be overwhelming trying to determine the source of your plant’s symptoms, but determine you must; identifying the root of the issue is the first step in healing your plant.

Plant ailments are the result of either abiotic factors (nonliving environmental stressors), biotic factors (living organisms), or most often a combination of the two. Fungal species are among some of the most common biotic agents of plant disease. Fungi are multi-celled organisms whose sprawling family tree comprises one of the six kingdoms of life. Some examples of organisms in the fungi kingdom are molds, mushrooms, mildews, and even yeast.

With some exceptions, fungi rarely attack healthy plants. Fungi are more likely to take up residence on a plant that has been previously weakened, as it is an easy target. Plants can be weakened by environmental factors (freezing temperatures, intense wind, too much direct sun, improper irrigation or pruning, etc.) that are out of sync with the plant’s preferred conditions. Exposure to certain chemicals can also damage plants. Non-native plant species are especially likely to be less vigorous than their native counterparts as they are rarely planted in environments that suit their native needs. Plants with low vigor are more susceptible to fungal infection.

This is an example of abiotic and biotic factors working in unison to destroy a plant host. Evolutionarily speaking, this relationship makes sense: any plant that is so ill-suited to a habitat as to become sickly is not a plant that should be living there. It is a weak link in the species gene pool and must be culled. This is where infectious fungal diseases come in. The fungi do their part to break down the susceptible tree and so prevent its unfavorable genes from reproducing and weakening the next generation of the species.

Understanding Mother Nature’s system of checks and balances helps shed some light on the importance of preventing disease by maintaining plant health.
Physical injuries can also make plants vulnerable to fungal infection. Oftentimes physical injuries occur during routine horticultural or landscape maintenance; pruning shears can slip and gash an unsuspecting trunk, clunky mowers can nick exposed roots, etc. While these injuries might seem minor enough, they can actually provide advantageous footing to windblown or waterborne spores looking to colonize a new host.

Phytophthora is a genus of fungi in which there are around 60 individual species. This fungal genus causes lethal root rot and has been in the spotlight since the 1840s, when Phytophthora infestans caused mass starvation and emigration in Ireland. Thankfully, since the Irish Potato Famine there hasn’t been a Phytophthora epidemic of the same scale but other species have been known to pop up and cause damage from time to time. Phytophthora cinnamomi is one such problematic strain.

P. cinnamomi is an aggressive species with a worldwide distribution, though it is suspected to have evolved in eastern Australia. Because of its expansive geographical range the fungus has many common nicknames including “cinnamon fungus” and “jarrah dieback.” P. cinnamomi has become especially problematic in western Australia, where it is listed as a ‘key threatening process’ by the federal government due to the fact that it threatens several plant species with extinction.

This root rot fungus prefers woody, perennial plants but has a recorded hit list of over 2,000 host species including fruits, vegetables, and ornamentals. Among those species affected are eucalyptus, avocado, pineapple, azalea, juniper, pine, and rhododendron just to name a few.

Phytophthora are part of the oomycete fungi group, also known as water molds. Oomycete fungi spread from plant to plant through water and so are most prevalent in moist environments with poor drainage. Phytophthora species first target the root system meaning that the plant becomes unable to obtain sufficient amounts of water and nutrients.

This also means that the initial symptoms of a Phytophthora infection are oftentimes confused with those of drought stress. This is a dangerous assumption and goes to show why careful observation and proper diagnosis of a plant’s symptoms is paramount; additional watering of the host plant will not cure these “drought” symptoms but instead provide the Phytophthora fungus with an ever more favorable environment in which to propagate. Additional symptoms are stunted growth and a blackening of the stem in herbaceous hosts or seedlings.

Phellinus noxius is a fungus in the family Hymenochaetacheae. Like Phytophthora strains, Phellinus noxius attacks the root system of its plant hosts. Common names include “brown tea root disease” and “brown root rot.” P. noxious usually prefers woody plant hosts and has a huge host range, affecting hundreds of plant species in the gymnosperm group (cone-producing trees) and both divisions of angiosperm (flowering plants).

The fungus is found in tropical and subtropical regions around the world. Unlike Phytophthora it thrives in dry, sandy soil, as flooding lessons the viability of dormant spores. P. noxius spreads from tree to tree via myceliar contact with the roots underground. The initial symptoms of a P. noxius infection are similar to those of Phytophthora: the leaves will exhibit the yellowing and subsequent wilting indicative of dehydration caused by dieback. Unfortunately these visible symptoms don’t develop until much after initial inoculum of the plant host; by the time your tree starts wilting the bulk of the damage has already been done to the roots.

In environments with strong winds plant hosts have been known to topple over, their underground support system having been decimated. In advanced cases the mycelium (vegetative body) of P. noxius can be seen aboveground, spreading from the roots up into the collar and trunk of the tree.

Stopping the cycle of infection is a daunting but necessary process. Most often the source of primary inoculum for a new P. noxius plant host is a piece of previously-infected root. Naturally, the first order of business is the removal of said diseased plant matter. This means making sure that each and every bit of the infected host plant is removed from the soil and destroyed. Chemical treatments with soil fumigants and ammonia have been cited as effective in the past.

The next step is replanting. In the early stages after soil treatment it’s a good idea to plant a vigorous species of ground cover, which will jump start the decomposition of any mycelium remnants lying dormant in the soil. Because dormant spores can remain viable in the soil for years, it is not advisable to ever replant any susceptible species in soils known to be previously diseased. Installing native, disease-resistant species is the final step in fungal management, and will discourage further propagation of the disease.