Plant Disease Management

Plant Management
Unlike insect and weed pests, most plant disease pests are so small they can't be seen with the naked eye. These tiny pests often go undetected until plants are already infected and symptoms, the outward signs of the infection, begin to appear.
All too often, plant diseases are "managed" by keeping plants covered with protective pesticides. This is costly and unnecessary. There are safer, healthier and less expensive ways to prevent crop losses to disease.
Overall, about 12% of the world's potential crop production is lost to plant disease pests. These pests include viruses, fungi, bacteria and nematodes (microscopic work-like organisms). They also include parasitic plants, such as dodder and mistletoe, which "feed" on susceptible plants.
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management information:
The Plant Disease Triangle

Why don't all untreated plants become diseased? In order for plant disease to occur, three major events have to happen in the same place at the same time, forming the "disease triangle." 

  • The plant must be present and be of the right age and variety to become infected. Sometimes, nutritional deficiencies can make a plant more or less susceptible to attack by disease-causing organisms. 

  • The pathogen must be present in sufficient numbers and strength to cause an infection. Without enough strong and "healthy" disease-causing organisms, infection can't occur. 

  • The environment must be favorable for disease. Even if the pathogen and the plant come in contact, disease may not result if the weather is too hot or too cold, too wet or too dry for the pathogen to penetrate the plant's defenses and cause infection and symptoms.
Plant Disease Management Options

There are many options for breaking the disease triangle and preventing disease. Chemical pesticides that weaken or kill disease-causing organisms should be the last line of defense, used only when necessary and precisely on time. 

Most of your options for managing disease are available before disease problems occur. If you use these effectively, you won't be forced into considering more limited options available after infection occurs.

What does all of this have to do with IPM?

Scientists observing the relationship between the incidence and severity of certain plant diseases and specific weather conditions have developed a predictive "model" of the process. Example: Apple scab is a disease caused by a fungus. By measuring air temperatures and the duration of the period leaves are wet, we can predict whether or not an infection has occurred. Thus, unnecessary fungicide applications can be avoided. 

When infection does occur, growers know to apply fungicide at the right time to achieve the best control.
The Modified Mills Chart

In the 1940s, a professor of plant pathology named Robert Mills studied apple scab and discovered much about the infection process. Mills and his colleagues boiled down the process into a chart for growers to use to decide if a fungicide treatment is warranted. A grower simply has to monitor how long leaves stayed wet and what the average air temperature was over the wetting period. By comparing observed values to those on the chart, a reliable spray decision can be made. Mills' Chart has withstood the test of time, and is still used today with slight modifications. The Mills Chart forms the basis for most computer-operated software models used for predicting apple scab infections.

Modified Mills Chart

Temp. (°F) Hours of Leaf Wetness Required
(Risk of Infection)
(Risk of Infection)
(Risk of Infection)
32 to 41 37 48 73
43 26 34 51
44 21 27 40
46 18 23 34
48 15 20 30
50 14 19 28
52 12 17 26
54 11 16 24
55 10 15 22
57 10 14 21
59 9 13 20
61 9 12 19
63 9 12 18
64 9 12 18
66 9 12 18
68 9 12 18
70 9 12 18
72 9 12 18
73 9 12 18
75 9 12 19
77 10 14 21
Plant Management, field of crops