Collar rot is a fungal disease caused by Phytophthora spp., which is notoriously difficult to control and can severely impact lawns, trees, shrubs, and other plants. This disease, often overlooked until symptoms become extreme, is especially damaging because it attacks the root and lower trunk areas, causing significant stress to the plant and potentially leading to death.
Understanding collar rot is essential for homeowners who want to protect their landscapes and maintain a healthy lawn, which is why the Little John’s team has put together this guide to collar rot in Arizona!
Collar rot is a disease resulting from soil-borne Phytophthora fungi, a genus containing numerous species capable of infecting a wide range of plants. These fungal pathogens thrive in moist, poorly drained soils, making lawns and garden beds in wet climates especially susceptible.
The term "collar rot" is derived from the plant’s "collar," or the area where the roots transition into the main stem or trunk. When infected, this critical junction point begins to decay, interrupting the plant’s nutrient and water transport systems. Phytophthora collar rot is one of the most common causes of root and crown rot in trees and lawns worldwide, especially in temperate and subtropical regions.
Phytophthora species are known as "water molds" because they depend on moist conditions to germinate, spread, and infect plants. While many plant diseases can be controlled by keeping leaves dry, Phytophthora spreads through the soil, primarily affecting plants at the root level. Here’s a deeper look at the primary factors that contribute to collar rot infections:
Collar rot spreads primarily through soil and water movement. Phytophthora spores, called "zoospores," move through water-saturated soil or surface water, making it easy for them to spread to nearby plants or lawns. Even splashing water can move spores from one plant to another. This is why wet conditions, especially in poorly drained soils, contribute to outbreaks. These spores can remain viable for months or even years in the soil, infecting plants when conditions are optimal.
Human activities such as moving contaminated soil, transplanting infected plants, or even splashing water from infected areas can spread the spores as well. Homeowners can unknowingly spread collar rot through everyday gardening activities. For example, planting new shrubs or trees in soil that contains Phytophthora spores can introduce the disease to previously healthy plants. Contaminated tools, pots, or footwear can also transfer spores between infected and healthy areas.
In trees, collar rot begins at the base of the trunk, where roots meet the stem, known as the “collar.” Infected bark near the soil line may darken, appearing water-soaked or cracked. A sticky, dark substance may ooze from infected areas as the disease progresses, and leaves may turn yellow or brown, especially during drought conditions, as the tree’s compromised root system struggles to uptake water. Trees may show stunted growth, weakened branch structure, and even death of large sections if the disease becomes severe.
In lawns, Phytophthora collar rot primarily affects the root system, and the grass will appear patchy or yellow, mimicking signs of nutrient deficiency. Infected roots become brown or black and often rot away, reducing their ability to absorb water and nutrients. During hot weather or drought, infected areas may wilt or die, as the weakened root system can’t support healthy grass growth. Over time, entire sections of the lawn may die, leaving visible brown patches that expand if the disease progresses unchecked.
Preventing collar rot starts with good landscape management practices. A good and easy start to collar rot prevention is to simply clean gardening tools, pots, and footwear between uses to prevent disease transfer. Being mindful of the areas of your yard that could become vulnerable to fungal infection is always the first step in prevention, and the following tips will help ensure that your lawn stays healthy.
Phytophthora spores can survive in soil for years, which is why prevention is so important, even if no signs of infection are noticed. Once collar rot sets in, treatment options are somewhat limited, but early intervention can mitigate damage. Remember to stay vigilant, and employ the following remedies if signs of damage begin to appear.
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