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| Photo Credit: Lane Greer |
| Powdery mildew infects bigleaf maple leaves in late summer. |
Ever hear a plant sneeze or a tree cough? If a petunia had to say, “Ahhh,” where would its tongue be? Obviously, plant diseases are very different from animal illness. The most basic idea behind plant disease is that three things have to occur simultaneously. The graphic depiction of this is called the Disease Triangle, and it looks like this:
The host is the plant itself. Some plants can host any disease, while others only entertain specific ones. For instance, lilacs are highly susceptible to bacterial blight, a disease caused by a bacterium that also infects apple trees and pears, including Bradford pears – that ubiquitous tree found in every Wal-Mart parking lot (and everybody else’s parking lot, yard, street planting…you get the idea). Bradford pears are now so prevalent across the country (except in the Upper Midwest), that controlling bacterial blight on lilacs has gotten much tougher, since bacterial blight can find a host so easily. Another disease, cedar apple rust, can’t exist unless both apple trees and Eastern red cedars are present (although by “present” I mean within a mile or two of each other). | Add Photo to Journal |  | | Photo Credit: Lane Greer | | These cut roses will have a shortened vase life because they’re infected with botrytis (also known as gray mold). |
The pathogen is the disease. Diseases are most often caused by bacteria or fungi, which are found literally everywhere. But without the right host and the right environment, diseases are like Dracula in the sunlight – they can’t do any harm. (They can’t suck your blood either.) Pathogens can be very specific and infect only one or a handful of hosts, or they can be very broad and able to live on just about everything.
The environment includes the weather conditions needed for a pathogen to thrive. Bacterial blight of lilacs occurs in spring because the bacteria do well in wet weather and mild temperatures. Powdery mildew typically occurs later in summer, when temperatures are 70-80 degrees F during the day and nighttime humidity is high. Tomatoes and many other plants get late blight, which means that the disease occurs late in the season, so you’ll usually see symptoms in mid- to late summer. Identifying a disease can be tricky business, but there are lots of resources available to help you determine what diseases are attacking your plants, including numerous books with helpful color pictures. Try the American Horticultural Society’s Pests and Diseases: The Complete Guide to Preventing, Identifying and Treating Plant Problems (DK Adult). You can also call your local Cooperative Extension Service office.
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