There are few landscape subjects more frustrating than plants underperforming (or worse, declining). Everything seems to be going great until wet weather rolls in, and all the sudden you’ve got a plant that isn’t acting itself, is covered in something odd looking, or has spots on the leaves. While plants can be exposed to some bacterial and viral illnesses, their most common and damaging illnesses are generally fungal. If you were to draw a venn diagram for conditions that favor plant growth and conditions that favor fungal growth, there would be a lot of overlap. Fortunately, there are some precautions we can take to reduce the incidence of disease, but first, let’s chat about the three most common types of diseases we see.
Root Rots
Pathogens that create root rot are always present, waiting on favorable conditions. Rhizoctonia, phytopthera, and pythium varieties are exceptionally common, can develop symptoms quickly, and can sometimes be hard to cure. They are most common and prevalent in areas that have excessive soil moisture. Poor drainage combined with overwatering, exceptional rainfall, or poor water management create a weakened state for the root and crown of plants making them susceptible to these diseases.
Symptoms are easily misidentified as drought stresses. Once the root system becomes infected, its ability to absorb and move water is diminished. An inability to provide moisture presents itself similarly to having no moisture to provide. With a little digging, both literally and figuratively, it’s fairly easy to tell the difference though. Wet conditions can be a good first clue. If the weather has been wet, it likely isn’t drought stress. Segments of the plant declining instead of the entire plant could be an indicator that root rot is taking place. Lesions at soil level on the stem are good indicators of certain root rots. Lastly, root rot tends to have a distinct smell. Once you’ve experienced it, it’s hard to forget.
Leaf Spots
Leaf spot creating pathogens are also fairly common in Georgia landscapes. Their many species and permutations tend to look how you think they’d look, but vary based on the infected plant and pathogen creating the injury. Generally speaking, they are gray or tan spots surrounded by some sort of defined margin. With time, certain infections and species of plants may create an actual hole in the leaf where the tissue desiccated.
Outside of entomosporium leaf spot effectively eliminating red tip photinia from being a viable choice in our landscapes, leaf spot diseases typically aren’t fatal and can be managed with a couple of simple actions. The disease can live on fallen leaf material and spread from splashing. Spores can also travel on the air. Excess moisture on the foliage also contributes to the likelihood of disease development. Controlling those issues will help control Leaf Spot.
Powdery Mildew
White, powdery fungus on your leaves and stems can be really unsettling. Hot days followed by sticky humid nights can mean a quick onset of these fuzzy fungi. It’s not uncommon to find this disease of crepe myrtles, roses, or gardenias. This presents a challenge because there is hardly a landscape in metro Atlanta that doesn’t have at least one if not all of these plants.
Fortunately, while unsightly, powdery mildew is rarely fatal and generally curable. The fungus is sustained by sending fibers into the tissues it rests upon, and spread by spores carried on the wind and landing on neighboring plants. It is important to consider that sooty mold may present itself in a similar way, but is generally an indicator of aphids or whiteflies depositing honeydew on the surface of the plant tissues.
Controlling Fungal Infections
Integrated pest management is a concept fundamental to the suppression and curative control of all fungal infections. Often abbreviated to IPM, integrated pest management is the combination of mechanical practices, cultural practices, and chemical applications to control unwanted pests. This ensures maximal control with minimal secondary inputs like fungicides. Considering the pest triangle is key to understanding and controlling fungal pathogens. They need a susceptible host, the presence of the pathogen itself, and conditions favorable for development.
Host
Just because a plant can become infected doesn’t mean it will. In fact, healthy plants in areas of low disease pressure are far less likely to become diseased and more likely to recover from them. Choosing the correct plant for the correct area of a landscape is key. Does it have enough light? Does the area drain sufficiently? Can this plant grow to its potential without requiring exceptional pruning that may weaken it. Answering these questions and planting accordingly help to ensure the plant is healthy and happy.
Maintaining a plant’s health is also key. Was it planted correctly? Is it being fertilized appropriately? Does it have sufficient water? Has it been maintained well? Addressing these concerns will maintain the health of a well-chosen plant. A plant that has its core needs met, is thinned properly for air flow, and doesn’t crowd a structure will stave off disease much more effectively.
Conducive Conditions
This is a tricky one. We have control over some of these things, but not all of them. Only watering in the morning, for instance, is great because it reduces the amount of time that the foliage stays moist. Most fungal infections thrive on excess moisture on the foliage. By watering in the morning when the excess will evaporate off at some point soon after is ideal. Even more ideal would be a drip system that avoids foliar wetness altogether. The best laid plans, however, can be thwarted by rain.
Another key way we can influence the conditions is by ensuring the soil doesn’t stay overly saturated. Clay-based soils have tiny pores, and can be difficult to wet sufficiently as well as dry out. Making a concerted effort to only provide what the plant needs helps stave off infections like root rots. This can be thwarted by prolonged rains, downspouts depositing at the base of your shrubs, or something as unexpected as the condensation line of your air conditioning unit.
Cleaning up the debris at the base of your plants is also key. That doesn’t mean you can’t have mulch, but it does mean you should clear out fallen leaves. Many species of leaf spot and mildew can stay on this fallen debris, easily spread by moisture or air currents. Removing this material takes that potential off of the table.
The Pathogen
The disease causing agent is the one we are quickest to want to address. In the same way we often don’t consider an ailment in ourselves until we are sick, we don’t generally think of plant pathogens until they’ve infected our plants. Then it is the sole focus of our efforts. This section is when we consider fungicide intervention as part of the curative process.
We are fortunate to live in a time where smart people have developed products that will intervene in the processes of fungal pathogens with little to no effect on the host plant. Before spraying anything, however, it is important to confidently identify what pathogen you are trying to control so you don’t unintentionally spray something that will have little to no effect on the target.
After the causal agent is identified, it’s important to choose the appropriate fungicide, but you should also look beyond that to find out whether you should remove the afflicted tissues from the plant. Identify conditions that may have contributed to its development. Are these conditions outside of your control, or is there something that can be done to remediate? This helps control the pathogen itself, but also control the conducive conditions which is equally important.
