Plant Pathology Guidelines for Master Gardeners
Contact: Dr. Richard Raid
Oh No! A Quiz
Now it is time to see how much you have learned.
In the following pictures, you need to describe the symptom and determine if this is:
- Bacterial
- Fungal
- Viral
- Abiotic or Non-Pathogen-Induced Problem
Remember that control measures are routinely available for many diseases and once a problem has been pigeon-holed into one of these four categories listed above, similar control measures exist for many problems in that category.
Figure 77
Host - snap bean.
Figure 78
Host - cucumber.
Figure 79
Host - tomato.
Figure 80
Host - mango.
Figure 81
Host - tomato.
Figure 82
This strawberry plant is in the middle of a patch that shows a definite damage gradient from East to West (most severe damage at eastern end, no damage at western end). What caused it?
Figure 83
Host - snap bean.
Figure 84
Bean pods.
Figure 85
Host - tomato.
Figures 86 & 87
Host - tomato.
A cut stem from one of the above plants placed in water.
Figure 88
What happened to these tomato plants?
Figure 89
What may have caused this problem on avocado fruit?
Figure 90
What is the peculiar symptom seen at the base of this potted sunflower?
Figure 91
What is the symptom evident on the mango flower cluster on the right? What is the cause?
Figure 92
This is the last quiz question!
Note the more or less circular to oval area of dead plants in the middle of a tomato field. This problem appeared literally almost overnight around October 1st in southern Florida. The area is not low. All the other plants are perfectly healthy. All cultural practices are the same as those normally used.
Hopefully, this Web tutorial has sharpened your plant disease diagnostic skills and made you more aware of the role of plant diseases in the framework of plant health.
Answers
Figure 77
Symptom: Stem rot. Fungus (see sign of fungus growing in stems). This is white mold of snap beans.
Figure 78
Symptom: leaf spot (or possibly leaf blight). Bacterium (note watersoaking of underside of leaf). This is angular leaf spot of cucumber.
Figure 79
Symptom: stunting, off color, mottling. Virus. This is tomato mottle virus affecting stake tomato.
Figure 80
Symptom: leaf spot. Fungus. Leaf spots appear fairly dry. Fungus may be the best answer simply on the basis of the 85% rule. Since the leaf is from a mango tree, this is, not surprisingly, mango anthracnose.
Figure 81
Symptom: wilt; when the lower stem is cut across horizontally and the cut end immersed in water, no cloudy streaming is seen. Fungus. This is Fusarium crown rot of tomato.
Figure 82
This is herbicide injury. Herbicide applied in a walkway just east of the strawberry patch drifted in the prevailing easterly wind over the patch, creating the gradient in severity of the damage.
Figure 83
The symptom seen here is a leaf spot. It is bacterial. Although it doesn't look particularly watersoaked or greasy, the clumped nature of the spots suggest bacterial origin. This is because bacteria are primarily dispersed in splashing rain so that a group of bacterial cells all land with a water drop in approximately the same area. This is bacterial blight of snap bean.
Figure 84
The symptom, evident if you look closely, is a fruit spot. There are tiny, slightly raised black pimples on these bean pods (fruit). This is fungal. The disease is Alternaria spot of snap bean.
Figure 85
The symptoms are mottling, stunting, and misshapen leaves of tomato. The problem is definitely viral. The disease is tomato yellows, an aphid transmitted virus.
Figures 86 & 87
The symptoms here are primarily wilting with some necrosis (browning) of the tomato foliage. Fig. 87 shows the famous bacterial streaming, so the problem is, of course, bacterial. The disease is bacterial wilt of tomato.
Figure 88
Don't ignore the obvious! Someone ran over these poor tomatoes. We refer to this facetiously as "tire blight".
Figure 89
The symptom is probably best described as a fruit blight. But the name of the disease is more descriptive - avocado scab. It is fungal. This conclusion may best be reached by the process of the elimination. It is certainly not typical of either a bacterial or viral infection and remember that about 85 percent of all plant diseases are caused by fungi.
Figure 90
This is a gall or tumor. It happens to be a bacterial gall. The disease is crown gall of sunflower.
Figure 91
Using the strictly correct botanical nomenclature, the symptom is panicle blight (the flower cluster is a panicle). Flower blight would be fine. The disease is fungal. Because it is on mango, the disease is - you guessed it - anthracnose.
Figure 92
The problem is a lightning strike! This is actually what happened in this commercial tomato field. The stems of the plants near the center of the strike are hollow due to the extreme heat of the electricity.