Environmental
Many plant problems and deaths are the result of environmental conditions rather than insects or disease. This includes but is not limited to overwatering, underwatering, air pollution, herbicides, pollination problems, nutrient deficiencies, salt injury, scorch, lightning damage, and mechanical damage. In some cases these causes may be quite evident but in other cases the real cause may be hard to identify. In many cases the primary cause of plant decline may be an environmental condition and only after the plant became stressed and weakened did it become attacked by insects or disease. In these cases if the environmental problem had been addressed early, insects or disease would not have been a problem.
Other images

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Engine exhaust damage on arborvitae (Thuja)
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The bottom section of a shrub, such as, this yew (Taxus), will become bare if the upper branches are longer and shade out the lower branches.
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A callery pear (Pyrus) is a poor plant choice for this area; it at least should have been pruned to eliminate the poor crotch angles (typical in callery pears) that caused it to split.
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Tattered, wilted horsechestnut leaves (Aesculus) caused by high winds
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High winds caused the tender new leaves on this horsechestnut (Aesculus) to wilt and tatter
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A closer look at the tattered, wind-damaged leaves on a horsechestnut (Aesculus)
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An ear of corn (Zea mays) with undeveloped kernals due to poor pollination
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Fertilizer spilled on a hosta leaf
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Chemical burn on ostrich fern leaves (Matteuccia)
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Looking very similar to fungal tar spot of maples, these black tarry spots on a dogwood leaf (Cornus) were caused by paint dripping from overhead lines
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Close-up of black spot on dogwood leaf (Cornus) caused by paint dripping from overhead lines; note splatter to the upper right of the spot
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Underside of dogwood leaf (Cornus) that had black paint dripped on it from overhead lines
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Close-up of black spot on dogwood leaf (Cornus) caused by paint dripping from overhead lines
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Possible sunburn on Christmas cactus (Schlumbergera). Tested negative for fungi and 2 viruses (INSV and TSWV).
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Possible sunburn on Christmas cactus (Schlumbergera). Tested negative for fungi and 2 viruses (INSV and TSWV).
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Possible sunburn on Christmas cactus (Schlumbergera). Tested negative for fungi and 2 viruses (INSV and TSWV).
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Close-up of chemical burn on ostrich fern leaves (Matteuccia)
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Damage to azalea (Rhododendron) caused by paint spray
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Spots on azalea leaf (Rhododendron) caused by paint spray
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Close-up of spots on azalea leaf (Rhododendron) caused by paint spray
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Shrub rose 'Hot Cocoa' (Rosa) with rose flower proliferation. Cause is unknown but could be environmental (bullhead) or a virus or virus-like disease, such as, aster yellows on rose.
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Shrub rose 'Hot cocoa' (Rosa) with multiple buds coming from a single flower. Cause unknown, resembles aster yellows on rose but could be bullhead or another environmental problem
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High nitrogen, boron deficiency, virus or virus-like diseases are all possible causes of the proliferation of flower buds on this 'Hot Cocoa' shrub rosa (Rosa)
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Flower proliferation of unknown cause on 'Hot Cocoa' shrub rose (Rosa)
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This basil plant (Ocimum basilicum)has become spindly from insufficient light. Grown in adequate light, this plant would be much shorter, fuller, and more flavorful.
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Hot, dry air and lack of humidity in winter can cause leaf tips to curl, brown and sometimes drop, as on this lemon verbena (Aloysia triphylla). Place plants away from heaters and vents.
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Lawn grass grows poorly in shade and needs 6 to 8 hours of direct sunlight every day.
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Lawn grass does not grow in dense shade
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Chilling injury to Cuban oregano (Plectranthus amboinicus)
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Chilling injury to Cuban oregano (Plectranthus amboinicus)
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Cold injury to honeydew melons (Cucurbita).
G. Holmes, Cal Poly, Bugwood.org
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Cold injury (low temperature) on cucumber.
D. Doohan, OSU / OARDC, Bugwood.org