How to Spot Common Plant Diseases

Published by Bucks Country Gardens in Lawn & GardenPlants

Where do plant diseases begin? Most people think plant diseases emerge from one singular cause, but actually they are caused by what plant pathologists call the “plant disease triangle.” Basically, the “plant disease triangle” is three factors coming together: 1) a susceptible host plant, 2) a pathogen (fungi or bacteria that causes disease) and 3) a favorable environment. For example, if you have a tomato plant susceptible to Septoria leaf spot, the plant may not get the disease unless the environment is right. In this case, conditions have to be warm and moist for the plant to succumb to the disease.

If you can alter any one of the three factors of the plant disease triangle, you can help prevent disease. “Doctor” David Jones, our resident horticulturist and arborist, says, “Monitor your plants closely. Keep them as healthy and stress-free as possible. Watering and feeding plants correctly make them happy.” And, who doesn’t want a happy plant? David also recommends good sanitation in your garden. He says, “Always cut back your perennials. Remove discolored and old leaves that have fallen off to prevent disease.” Locating plants too close together can also encourage disease since there is less air circulation.

While David does not want you to diagnose and treat your plants alone, below is a guide to the most common plant diseases to help you be aware of the signs of disease. Bring a sample of any plant, sealed inside a plastic bag, to our Garden Pharmacy for proper diagnosis and treatment recommendations.

Powdery Mildew: This disease appears as powdery white or gray splotches on the upper surfaces of leaves, and can also be found on lower leaves, stems, flowers, buds and fruit. While it is not the prettiest disease to look at, it is usually not fatal. Most plants are susceptible to this disease with lilacs, cucumbers, crab apples, phlox, monarda, roses, grapes, squash and peonies being the most likely targets.

Rust: This disease mostly affects woody and herbaceous plants. It is identified by white, brown or orange spots (spores) forming on the underside of leaves with yellow spots appearing on the upper surface. While this disease is not usually fatal it does weaken plants and reduces flower production.

Leaf Spots: Especially common in perennials, trees and shrubs, this disease varies based on the host plant and type of pathogen involved. The disease starts with brown or black spots appearing on leaves, which can eventually combine to form blotches. Leaves may yellow and drop prematurely. 

Pythium Root Rot:  Oversaturated soil either from heavy rainfall, poorly drained soil or overwatering contributes heavily to this disease. The disease can cause the roots and stem of plants to turn dark brown, and the leaves to turn brown and yellow. Eventually the plant will die, especially if it is a seedling. 

Phytophthora Root Rot:  This disease primarily affects trees and shrubs planted in too wet of a location. Leaves turn brown or yellow and eventually wilt and drop off. Infected trees can survive a few years before succumbing to the disease fully. Oak, Dogwood and Rhododendrons are particularly susceptible to this disease.

“Doctor” David wants you to have happy, healthy plants, trees and shrubs. Feel free to bring in a sample cutting of your diseased plant in a sealed plastic bag and David, or any other of our knowledgeable Greenhouse and Nursery team members, will take a look at it and get it the “medicine” it needs to return to full health.


David Jones | Horticulturist, Arborist and Customer Service Specialist

Dalissa Reeder | Social Media Manager