Plant Diseases

How to Manage Plant Diseases

Text: Master Gardeners of Ottawa-Carleton
Reference Manual for Ontario Master Gardeners 10-12 3rd Edition, 2016

Anthracnose

A fungal disease caused by the fungi in the genus Colletotrichum, a common group of plant pathogens that are responsible for diseases on many plants. Anthracnose is used to describe diseases that result in a wide range of symptoms.

PLANTS AFFECTED

Many deciduous trees and shrubs can be affected. Host plants include allium such as onion, garlic and leek, pepper, tomato and potato, and miscellaneous others – beans, cucurbits, lettuce, strawberry and, raspberry.

EVIDENCE

Generally appears first as small and irregular yellow, brown, dark-brown, or black spots. The spots can expand and merge to cover the whole area. The color of the infected part darkens as it ages. Besides leaf spots, the disease can produce blotches, distortions, defoliation, shoot blight, twig cankers and dieback on many different deciduous trees and shrubs.

MANAGEMENT

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Anthracnose on xxxx

Anthracnose

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Anthracnose on tomato.

Black Knot of Plum and Cherry

Black Knot

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The fungus Dibotryon morbosum causes this potentially serious disease which affects both wild and cultivated species of cherry. Yield is reduced since severely infected trees are stunted and because control necessitates pruning to remove the knots.

DISEASE CYCLE

Winter spores formed in mature black knots are spread by wind and rain to twigs where infection takes place through unwounded tissue. Infection continues to occur until terminal growth stops and it is most severe when conditions are mild and wet. Only several months after initiation of infection do green swellings become visible and usually not until spring. These newly formed knots produce summer spores. The fungus extends several inches beyond the knots and knots will expand with age.

PLANTS AFFECTED

Black knot disease occurs on numerous cultivated and wild plums prunes and cherries (Prunus spp.)

EVIDENCE

Rough elongated hard black swellings on twigs but also on branches or stems are characteristic of this disease. Usually knots occur on one side of the twig but occasionally branches become completely girdled killing the portion above the infection. New infections show up as green swellings which enlarge develop cracks and turn black with age. Old black knots may be partially covered by a white to pinkish mold and be riddled with insect holes.

MANAGEMENT

Botrytis (Gray mold)

Gray mold (Botrytis cinerea) is one of the most common diseases affecting bedding plants. By changing cultural conditions so that they are unfavorable to fungal growth, the disease can be successfully controlled. Proper diagnosis is necessary prior to undertaking any control strategy. Because gray mold affects numerous host plants, care must be taken in examining affected plants. Symptoms of gray mold depend on both host and environmental conditions. Commonly observed symptoms include: bud blast, leaf spots, flower blight, stem canker, and/or crown rot. If conditions are severe, plant may die. Infected tissue is soft and brown and may appear water-soaked. A key sign of Botrytis infection is the proliferation of gray mold covering the diseased plant.

DISEASE CYCLE

Infection often begins at the site of the flower or bud. Flower blight on bedding plants is one of the first symptoms of this disease. The fungus establishes itself in the petals. If humidity is high and temperatures are warm, the fungus spreads from the flower into pedicel/peduncle. Eventually the fungus invades the stem, leading to plant death. Symptoms of flower or bud blast begin as irregular gray/brown spots on the petals, whereas buds turn brown and/or have a water-soaked appearance. Infected buds may not open, or they may fall off.

Gray mold

PLANTS AFFECTED

Hosts commonly include, but are not limited to: begonia, carnation, chrysanthemum, cyclamen, geranium, impatiens, marigold, petunia and zinnia. Vegetables and fruit are vulnerable, even in our fridges. Tropical house plants such as African violets, begonia, cyclamen. Gray mold will cause rots and spots on bulbs, corms, or tubers of many garden plants such as chrysanthemum, dahlia, geranium, peony, snapdragons, tulips, hyacinths, lilies, amaryllis and gladioli.

Gray mold

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Gray mold on xxxx

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Gray mold on xxxx

EVIDENCE

Gray mold often infects the plant through flowers or leaves. Buds and flowers hang down after being infected. It can present as light gray-brown cobwebby mold. Infected succulent stems and fruit become soft and watery, the epidermis cracks, becomes wrinkled, sunken and finally dry looking.

MANAGEMENT

Chlorosis

Many factors singly or in combination contribute to chlorosis – yellowing of leaf tissue due to a lack of chlorophyll. Some of the most common causes among trees and shrubs include nutrient deficiencies related to soil alkalinity (high pH), drought, poor drainage, and compaction of the soil.

One cause of chlorosis is a deficiency of iron or manganese, both of which are present but unavailable in high pH soils (pH>7.2). When leaves of plants become chlorotic, always determine the primary cause through a soil test.

PLANTS AFFECTED

Herbaceous plants as well as woody plants are susceptible to chlorosis. Common tree species exhibiting chlorosis are pin oak, red maple, white oak, river birch, tulip tree, magnolia, and white pine.

EVIDENCE

New growth on plant appears washed out faded light green or yellowish in color that remains into maturity. The faded out new growth will have dark green leaf veins. The older established growth will be a normal healthy vibrant green color.

Chlorosis

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MANAGEMENT

Early and Late Blight

Late Blight on Tomato

Late blight lesion on a tomato leaf appears as a brown patch surrounded by a light green or yellow halo.

Water soaked greasy blotches develop on the top or along the sides of infected tomato fruit eventually becoming sunken and developing into a dry brown rot. 

Lesions on tomato stems caused by the late blight pathogen appear as dark brown patches that eventually expand and girdle the stem.

Early blight, is caused by plant pathogen, Alternaria solani, in solanum species. The pathogen produces distinctive “bullseye” patterned leaf spots and can also cause stem lesions and fruit rot on tomato and tuber blight on potato. Late blight is a potentially devastating disease of tomato and potato, infecting leaves, stems, and fruits of tomato plants. The disease spreads quickly and can result in total crop failure if untreated. Late blight of potato was responsible for the Irish potato famine of the late 1840s. Late blight is a potentially devastating disease of tomato and potato, infecting leaves, stems, and fruits of tomato plants. The disease spreads quickly and can result in total crop failure if untreated.

DISEASE CYCLE

Phytophthora infestans (late blight) can overwinter in potato cull piles. Overwintering in tomato fruits, it may give rise to infected volunteer seedlings the following season. The most common routes of introduction each season are infected potato seed tubers, infected tomato transplants shipped in from other regions, or windblown sporangia (asexual spores). Under cool, wet conditions, P. infestans can produce thousands of sporangia per lesion in less than five days. These sporangia easily become air-borne, resulting in prolific spread of the pathogen. Long-distance spread to other fields is also likely, particularly under cloudy conditions.

PLANTS AFFECTED

There are several strains of the pathogen, some that infect potatoes, some that infect tomatoes and many that infect both crops.

EVIDENCE

Symptoms first appear as water soaked light green to grey lesions on leaves and stems of tomato and potato plants. During cool (18-25°C), wet or humid conditions the lesions expand rapidly and often appear as a brown necrotic spot surrounded by a light green or yellow halo.  

MANAGEMENT

Lawn Disease and Pest Management

Disease and pest management begin with prevention using good cultural practices and disease resistant grass species and cultivars that are appropriate for site conditions. Blends of species and cultivars are more resistant to pests and pathogens than single varieties. Inter-planting with non-grass species further reduces the probability of infection or infestation. However, even with the best of care, lawns can develop issues.

Regular monitoring of lawns for signs of problems is important. Early detection and identification of stress, disease, weeds or pests can prevent serious health issues and infestations from developing. Beyond changing cultural practices when problems arise, management practices can include mechanical or physical controls, biological controls and chemical controls. 

In 2009, the Ontario government banned use of most pesticides on home lawns. Only a small selection of Class 11 pesticides remains available. 

Helpful pesticides (fungicides, insecticides and goose repellent) that are allowed for cosmetic uses on home lawns include: 

Leaf Galls

Leaf Gall

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Leaf Gall

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Leaf galls are small ‘pimples’ on the leaves sometimes in mass numbers. The small bumps caused by insects or mites that were feeding on the leaf or caused during egg-laying activity typically during late spring. The insect or mite either damaged the plant material or produced a salivary secretion that increased the production of normal plant growth hormones.

There are a variety of insects or mites that cause galls to form including: Eriophyid mites, Psyllids or jumping plant lice, aphids, adelgids, gall wasps, and gall midges. Some of these pests target specific plant species others impact numerous plants.

PLANTS AFFECTED

Many trees, shrubs, and plants including maple oak, roses, rhododendron, and goldenrod can be affected.

EVIDENCE

Small ‘pimples’ on the leaves sometimes in mass numbers or deformities on plant parts can be observed.

MANAGEMENT

Maple Tar Spot

Tar spot is a disease caused by fungi in the genus Rhytisma and gets its name because it looks like droplets of tar on leaf surfaces. In general tar spot alone will not harm the long term health of the tree but is aesthetically unappealing and, in severe infections, can cause premature leaf drop.

DISEASE CYCLE

The fungal spores overwinter on infected leaves that fall to the ground. In spring those spores mature and travel up to new growth by wind or water. The first symptoms of infection by a tar spot fungus usually show up in mid-June as small (less than 1/8 inch diameter) pale yellow spots. The spots enlarge and their yellow colour intensifies as the season progresses. On red maple and silver maple a black spot usually develops in each yellow spot by mid-July to early August. The black spot grows in diameter and thickness until by late summer it truly does look like a spot of tar. The surface of the spot may have a pattern of wavy indentations or ripples.

Another form of tar spot affects striped and Norway maples. On these trees 20 to 50 small spots each no larger than a pin-head appear in late July or early August. On striped maple the spots do not enlarge much after they first appear. On Norway maple however the spots grow and eventually coalesce to yield a larger black mass up to 1 & 1/2 inches in diameter. The surface may be slightly roughened to smooth but will not be rippled. The fungus may also attack the seeds of maple.

PLANTS AFFECTED

Maple species most commonly red, silver, and Norway.

EVIDENCE

Starts with tiny black dots on the leaves on maples in June. These dots merge together to make a tar spot. Moist rainy 20 degree weather is ideal for spread of the fungus.

Maple Tar Spot

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MANAGEMENT

Powdery Mildew

Powdery Mildew example

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Powdery mildew on xxxx

Powdery Mildew example

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Powdery mildew on xxxx

Powdery mildew is a widespread fungal disease affecting both vegetative and woody plants. While some trees and shrubs are more susceptible, almost any woody plant can be infected under the right conditions (favoured by shading and high humidity). Typical hosts include lilac, caragana, crabapple, chokecherry or rose.

DISEASE CYCLE

The pathogen over-winters on fallen leaves, with spores spreading during wet weather in spring. Following the initial infection, spores are continuously produced and spread by wind and rain to further infect other leaves during warm, humid, summer weather.

PLANTS AFFECTED

Woody plants such as Azalea, Honeysuckle and Viburnum. Herbaceous plants such as Chrysanthemums, Phlox, Shasta Daisy, Zinnia, Squash and Zucchini.

EVIDENCE

White fuzzy spots or patches on leaves, stems, fruit or flowers, primarily on the upper side of the leaf. Spores are spread by wind. Symptoms are most common in late summer or early fall. It rarely kills the host plant however the symptoms are unsightly and diseased plants may be more prone to overwinter injury.

MANAGEMENT

Wind-Burn and Leaf-Scorch

Wind Burn: A condition in which foliage first browns around the edges and in bad cases browns all over. In severe cases the plant can drop leaves or graduate from winter-burn to dead. In winter evergreen plants are prone to wind scorch (also called leaf scorch). This is caused by cold winds and poor soil conditions resulting in scorched brown dry leaves.

PLANTS AFFECTED

Evergreen shrubs, broadleaf evergreens trees, and tall perennials.

EVIDENCE

Dried leaf margins (can also indicate fertilizer burn)

MANAGEMENT

Wind Burn

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Leaf Scorch: Scorch most often occurs following prolonged periods of dry windy weather or bright sunshine when the roots are unable to supply water to the foliage as rapidly as it is lost by transpiration from the leaves. In mild cases the leaves remain attached and little damage results. In more severe cases plants may drop leaves prematurely although such plants do not die. Scorch can reduce the health of a plant making it more susceptible to attack by insects and diseases.

PLANTS AFFECTED

May occur on any species of tree shrub or herbaceous plant moved outdoors that are not used to the extra UV rays will burn. Seedlings are very susceptible.

EVIDENCE

Browning of leaf margins and/or yellowing or darkening of the areas between main leaf veins. Leaves may dry turn brown and become brittle. Premature dropping of leaves and twig dieback may occur during the late summer.

Narrow leaf evergreens display brown or purple brown discolouration of the needle tips. If unfavourable conditions become more severe browning of needles increases. This should not be confused with the browning and shedding of older interior needles. Scorch may result from hot dry weather or from strong dry winter winds when the ground is frozen. Symptoms may not become apparent for a month or more after the initial injury.

MANAGEMENT

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