Plant Pathology Guidelines for Master Gardeners
Contact: Dr. Richard Raid
Most plant pathologists concentrate on those problems caused by parasitic organisms (primarily microorganisms). However, it is important to recognize non-parasitic disorders of plants so that these can be differentiated from plant diseases when Master Gardeners are confronted with clientele problems.
These non-parasitic disorders include:
- Air Pollution
- Genetic Defects
- Mechanical Injury
- Nutrient Imbalances
- Temperature Extremes
- Toxic Chemicals
- Water Imbalances
Most of the examples used throughout this tutorial will come from our 23 years experience with diseases of commercial vegetables in Florida. This is our primary area of responsibility. Therefore, naturally, our collection of photographs reflects this activity in the execution of our daily tasks. However, the principles of plant disease diagnosis are applicable to all situations, be they a 500-acre commercial tomato field in southern Florida or an annual flowerbed in Pensacola.
Each figure below is associated with a question. See if you can select the correct answer.
What caused this widespread damage to this snap bean field in Homestead?
Hint: The damage appeared rapidly on January 13th of the winter vegetable season.
- Spray drift from an adjoining orchard
- A lightning strike
- A rare genetic disorder
What is the common name of this problem on the tomato fruit?
Hint: The damage is always on the outside of the canopy.
- Phosphorous deficiency
- Weather fleck
What is the cause of darker green flecks or spots on this immature tomato fruit?
Hint: It is on all fruit surfaces.
- Unusually dry weather
- A genetic disorder
- Fertilizer burn
What is this problem on ficus?
- Potassium deficiency
- Excess water
- Frost injury
- Mechanical injury from leaves rubbing in the wind
In what plant organ did this problem originate?
Examine this picture. These potato plants showed this brown flecking on leaves almost overnight. The entire field is uniformly affected. The location of the field is in Homestead, Florida in southeast Miami-Dade County. It occurred in early November.
Select the most probable cause for this problem.
- Frost injury
- Air pollution
- Metribuzin herbicide injury
- Excess of boron
If you selected "d" (frost injury), you are correct! An important point to note is that the damage is quite uniform throughout the field. Most environmentally induced problems tend to be uniform, whereas parasitic diseases may begin as spots throughout the field. Of course, you can check the weather and see that frost did indeed occur just prior to the widespread appearance of this problem.
If you selected "b", you are correct. Fruit exposed to long periods of bright, hot sunlight may show "scalded", shriveled, and dried tissue, known as "sunscald" or sometimes "sunburn". It appears only on the side of fruit facing to the outside of the canopy, where the tissue is exposed to the ravages of the sun.
The correct answer is "c". This is a genetic disorder called "gold fleck". It is caused by a "bad" gene that has been carried in this tomato line and is expressed in some fruit under certain conditions.
"a" is the correct answer. There are a myriad of symptoms associated with mineral deficiencies and excesses. Proper identification usually requires considerable experience with specific plants.
If you answered "c", give yourself a pat on the back. This is metribuzin herbicide injury to tomato. The herbicide entered the tomato roots and was transported to the main veins of the leaf where it destroyed the chlorophyll, causing the "vein-clearing" seen in the picture.
"b" is the correct answer. Automobile-based air pollutants, such as ozone, sweep down into south Miami-Dade county from Miami with the first substantial north-to-south cold fronts of late fall. Potatoes are extremely susceptible to these air pollutants. This malady is sometimes referred to by growers as "weather fleck."