What are the common problems with the plant?
Another good diagnostic perspective is to consider a plant’s common problems. All plants have their own set of diseases, insect problems, and cultural dilemmas; there are no problem-free plants. Pondering these common quandaries can create somewhat of a bias, especially if you are seeing something new, but it helps rule certain problems out.
For example, fire blight, caused by the bacterium Erwinia amylovora, causes a blighting of shoots that result in discolored leaves and curling of the shoot often characterized as a “shepherd’s crook.” This symptom is helpful in considering fire blight as a possibility. However, such symptoms can also be caused by many plants by far simpler problems, such as moisture stress, resulting in leaf and shoot wilting. For which plants should fire blight be considered a possibility? As it turns out, fire blight occurs only on plants in the rose family (Rosaceae). So if you see a crabapple, firethorn, or mountain-ash with a shepherd’s crook symptom, fire blight should be considered and investigated. If the plant is a maple, white ash, or pine — not members of the rose family — fire blight is not a possibility.
Some plant problems are specific to a particular genus. Native ash trees (Fraxinus spp.) in North America are under threat from the non-native emerald ash borer (Agrilus planipennis). Ash trees are members of the olive family (Oleaceae); however, other members of this family, such as lilacs and forsythia, are not threatened by the borer. As noted above and in question #1, since mountain-ash belongs to the rose family, it may suffer from fire blight but it will not be attacked by emerald ash borer. Stinking-ash does not belong to the rose family, nor is it true ash, so the plant dodges both the fire blight and emerald ash borer bullets.
Be aware that some plant pathogens and plant pests have alternate hosts, so two very different plant groups may be affected by the same problem in different ways. The fungus Gymnosporangium juniper-Virginia produces large, reddish-brown plant galls that sprout fungal horns on the stems of junipers. On apple leaves, the fungus produces round, lipstick- red leaf spots. The common name for this disease reflects the two hosts: cedar-apple rust. The “cedar” refers to eastern redcedar (Juniperus virginiana) which is a juniper, not a true cedar as indicated with the contraction used in the common name. Once again, beware of common names!
Fungi are not the only gall-makers. There are a number of insects and mites that direct the growth of plant galls, and most are highly specific to their hosts. Indeed, most are so specific the gall-maker can be identified to species based on the gall structure alone without the need to see the actual gall-maker. There are over 800 gall-making insects that are specific to oaks, and over 700 are tiny wasps belonging to the family Cynipidae. The cynipid wasp Amphibolips prunus stimulates plant cells in pin oak acorn caps to grow an unusual ball or plum-shaped structure that surrounds a single wasp larva. The galls are commonly called oak plum galls, or oak acorn galls, and they are only produced by this wasp on pin oaks.
Knowing your plants, and even what family and genus they are in, is a great starting point for diagnostics. This, of course, helps not just with identifying infectious diseases like fire blight and rust, but with other problems as well. Consider a yew or rhododendron growing in poorly drained soil. Knowing these plants are particularly prone to root decline and root rot in poorly drained sites helps immensely with a proper diagnosis when plant decline is evident. It should not blind you to other possibilities, but it certainly is the type of smoking gun that should be investigated.
For example, fire blight, caused by the bacterium Erwinia amylovora, causes a blighting of shoots that result in discolored leaves and curling of the shoot often characterized as a “shepherd’s crook.” This symptom is helpful in considering fire blight as a possibility. However, such symptoms can also be caused by many plants by far simpler problems, such as moisture stress, resulting in leaf and shoot wilting. For which plants should fire blight be considered a possibility? As it turns out, fire blight occurs only on plants in the rose family (Rosaceae). So if you see a crabapple, firethorn, or mountain-ash with a shepherd’s crook symptom, fire blight should be considered and investigated. If the plant is a maple, white ash, or pine — not members of the rose family — fire blight is not a possibility.
Some plant problems are specific to a particular genus. Native ash trees (Fraxinus spp.) in North America are under threat from the non-native emerald ash borer (Agrilus planipennis). Ash trees are members of the olive family (Oleaceae); however, other members of this family, such as lilacs and forsythia, are not threatened by the borer. As noted above and in question #1, since mountain-ash belongs to the rose family, it may suffer from fire blight but it will not be attacked by emerald ash borer. Stinking-ash does not belong to the rose family, nor is it true ash, so the plant dodges both the fire blight and emerald ash borer bullets.
Be aware that some plant pathogens and plant pests have alternate hosts, so two very different plant groups may be affected by the same problem in different ways. The fungus Gymnosporangium juniper-Virginia produces large, reddish-brown plant galls that sprout fungal horns on the stems of junipers. On apple leaves, the fungus produces round, lipstick- red leaf spots. The common name for this disease reflects the two hosts: cedar-apple rust. The “cedar” refers to eastern redcedar (Juniperus virginiana) which is a juniper, not a true cedar as indicated with the contraction used in the common name. Once again, beware of common names!
Fungi are not the only gall-makers. There are a number of insects and mites that direct the growth of plant galls, and most are highly specific to their hosts. Indeed, most are so specific the gall-maker can be identified to species based on the gall structure alone without the need to see the actual gall-maker. There are over 800 gall-making insects that are specific to oaks, and over 700 are tiny wasps belonging to the family Cynipidae. The cynipid wasp Amphibolips prunus stimulates plant cells in pin oak acorn caps to grow an unusual ball or plum-shaped structure that surrounds a single wasp larva. The galls are commonly called oak plum galls, or oak acorn galls, and they are only produced by this wasp on pin oaks.
Knowing your plants, and even what family and genus they are in, is a great starting point for diagnostics. This, of course, helps not just with identifying infectious diseases like fire blight and rust, but with other problems as well. Consider a yew or rhododendron growing in poorly drained soil. Knowing these plants are particularly prone to root decline and root rot in poorly drained sites helps immensely with a proper diagnosis when plant decline is evident. It should not blind you to other possibilities, but it certainly is the type of smoking gun that should be investigated.
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