I have oak trees on my property and if I'm checking my trees for root rot by looking for spongy, shredding wood, I'd be wrong. Many types of root rot form no structures and are invisible to the human eye. And if I don't associate root rot with dead tomato, eggplant and pepper plants, I would be wrong again.
The pathogens causing root rot live in the soil and can affect shrubs, trees, citrus and vegetables. Root rot is caused by an adaptable pathogen.
One of the most common root rots in Marin County is caused by organisms in the genus Phythopthora. These pathogens are called "water molds," but they behave like fungi. They spread via swimming spores typical of waterborne molds, but they have evolved to live on land plants. There are hundreds of Phythopthora species and they have more than 1,000 host plants aiding their proliferation.
Phythopthora spores need water contact to infect a plant. Wet soil around the base of a plant or standing water can easily serve as this medium. Water can be delivered to the organism via irrigation systems that are watering directly on the root system and by overwatering, especially in warmer seasons. Also planting too deeply, landscaping with insufficient drainage or burying the crowns of trees can encourage root rot.
Once a plant is infected, the pathogen kills the roots and moves up the plant's water and nutrient uptake system to the crown. All of this happens underground so it is invisible to even an observant gardener. And ironically, the symptoms of root rot look similar to drought stress. The plant begins to decline, with wilting leaves turning color.
Other pathogens may be present in the infection, decaying the wood and making it spongy. If it is mainly Phythopthora, the bark may be discolored but the wood will be solid. The rate of decline and eventual death of the plant is determined by the size and age of the plant and how rapidly the pathogens can spread from the roots to the crown.
So what can a homeowner do to deter this pest? The first line of defense is to adjust irrigation for plants so that there is sufficient opportunity for the soil to dry around the plant before watering again. If you've planted birch trees or alders, which like their feet wet, you will be challenged trying to reach a balance that keeps the trees healthy. If you suspect a Phythophthora infection, contact your arborist to evaluate whether a fungicide would give the plant some protection.
The keys to minimizing the chances of getting root rot are good water management and sufficient drainage. Buy your plants from a reputable source so you're not importing the pathogen. Good sanitation practices can help you avoid spreading the pathogen from one area of your garden to another via water or soil.
There is more information on the Master Gardener website at www.marinmg.org. Click the link to "Pests and Other Problems" on the left hand column of the home page.