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Alternaria Leaf Spot -- Alternaria alternata, Alternaria spp.
Host: Calathea, impatiens, marigold, pittosporum
Symptoms: Alternaria leaf spot first appears as water-soaked spots that are less than 1 mm in diameter. Eventually, the spots turn reddish-brown to black in color and become somewhat circular in shape. While the spots can enlarge up to 2 mm in diameter, they usually do not coalesce.
Conditions Favoring Disease: This disease often occurs where overhead irrigation is used and with exposure to rainfall.
How Pathogen Survives/Disperses: Conidia are dispersed to new leaves by irrigation or rain splash. Seed-borne disease is also common.
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Alternaria Leaf Spot -- Alternaria panax
Host: Dizygotheca, Fatshedera spp., Fatsia, Polyscias spp. (Aralia), schefflera
Symptoms: This disease causes large, irregularly-shaped dark-brown spots to form on the leaves of plants and sometimes on their petioles and stems. Leaf drop commonly occurs with severe infections, giving the foliage a sparse appearance that is easily misdiagnosed as a symptom of root rot.
Conditions Favoring Disease: This disease is favored by wet conditions and temperatures between 65° and 86°F. Severe infections occur at temperatures between 75° to 80°F.
How Pathogen Survives/Disperses: The conidia of A. panax are carried by splashing water from fallen leaves on the ground to leaves several feet above the ground.
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Helminthosporium Leaf Spot -- Bipolaris spp.
Host:palms, ficus, assorted foliage
Symptoms: This disease causes reddish-brown to black spots, approximately 2- to 5-mm long, all over the leaf surface. Yellow margins typically appear around the spots on palms. These spots coalesce under optimum conditions, forming irregularly shaped necrotic areas on leaf tips and along the margins.
Conditions Favoring Disease: This disease is favored by overhead irrigation and exposure to rainfall. Plants exposed to full sun are more severely affected by this disease than those produced with partial shade.
How Pathogen Survives/Disperses: Healthy plants can be easily infected by placing diseased plants in the same greenhouse or area. Splashing water, commonly produced by overhead irrigation, can easily dislodge large numbers of conidia and disperse them onto healthy tissue. Conidia can also be distributed by wind-driven rain.
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Cercospora Leaf Spot -- Cercospora spp.
Host: Cordyline, dracaena, ficus, hosta, hydrangea, pansy, pittosporum, succulents
Symptoms: Cercospora leaf spot on some foliage plant hosts appears as tiny, slightly raised red or dark green spots that appear on the lower leaf surfaces. On Ti plants rust-colored specks form initially then enlarge into elliptical areas up to 7 mm across and turn tan to brown in color. On pansies, small purplish spots enlarge into conspicuous black spots with fringed margins 1/4 inch or more in diameter. Other symptoms include angular leaf spot.
Conditions Favoring Disease: Cercospora leaf spot is associated with heavy rainfall or with the use of overhead irrigation.
How Pathogen Survives/Disperses: Spores are spread by splashing water. Extended periods of leaf wetness allow spores to germinate and infect the foliage.
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Anthracnose, Colletotrichum Leaf Spot -- Colletotrichum gloeosporioides, Collectotrichum spp.
Host: Many species of herbaceous plants, Euonymus spp.
Symptoms: Anthracnose is one of the most common plant fungi in tropical or temperate regions. Depending on the plant species, symptoms can appear as small, water-soaked speckles or large necrotic and chlorotic lesions that are circular to irregularly shaped. Lesions are yellow, tan, brown or black in color, often with a bright-yellow chlorotic halo. Infected leaves sometimes abscise. On some hosts, the spots can be angular in shape due to their confinement between leaf veins, while on other hosts a frog-eye spot can develop. Flowers may also be affected.
Conditions Favoring Disease: This disease may occur in areas with overhead irrigation and rainfall and where foliage does not dry rapidly. On some plants, Anthracnose occurs after heat or water stress.
How Pathogen Survives/Disperses: Spores are produced within an acervulus (a spore-producing structure). The fungus survives in spore structures within acervuli, in or on recently killed tissue. Spores are dispersed by water-splashing (i.e., irrigation, rain).
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Corynespora Leaf Spot -- Corynespora cassiicola
Host: Aeschynanthus, aphelandra, Coleus spp., ficus, African violet
Symptoms: Corynespora leaf spot appears on most plants as tiny, sunken, brownish lesions that enlarge and darken with age. The leaf spots are dark brown to black in color and appear wet on aphelandra, while on ficus, the spots are small to large and reddish in color and appear on the most recently mature leaves. Leaf abscission may occur when leaf spots expand between the veins.
Conditions Favoring Disease: This disease typically is favored by conditions with excessive water, including condensation on the plant overnight.
How Pathogen Survives/Disperses: Spread via rain/irrigation splash.
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Cylindrocladium Root Rot, Leaf Spot and Stem Canker -- Cylindrocladium spp.
Host: Azalea, heliconia, rose, and spathiphyllum
Symptoms: Yellowing of lower leaves and wilting are indications of root rot. Cylindrocladium stem cankers cause a brown, internal discoloration of the stem of azalea. The vascular browning extends to approximately 1 inch above the soil line. Angular leaf spots, bounded by leaf veins, are caused by Cylindrocladium on azalea. Defoliation may occur. Root and petiole rot are common on spathiphyllum.
Conditions Favoring Disease: The disease on azaleas occurs only in greenhouses and is favored by high humidity and temperatures of approximately 85°F.
How Pathogen Survives/Disperses: The conidia are easily carried on hands and by wind and water. Water-dispersed conidia from fallen infected leaves can also wash into the soil where they can cause root rot. Microsclerotia survive in soil for years.
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Dactylaria Leaf Spot -- Dactylaria humicola
Host:Philodendron spp.
Symptoms: Dactylaria leaf spot first appears as small, water-soaked areas on both surfaces of young leaves but symptoms are more severe on the lower surface. As the spots mature, they turn yellow to tan in color and commonly have sunken centers. Only the immature leaves of heart-leaf philodendron are susceptible to the pathogen.
Conditions Favoring Disease: This disease is favored during the summer months and occurs primarily on ground bed plantings of the host.
How Pathogen Survives/Disperses: The fungus survives in soil and spreads to plants grown in ground beds.
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Iris Leaf Spot -- Didymellina macrospora (Heterosporium iridis)
Host: Iris
Symptoms: Iris leaf spot causes spots that are small and grayish-brown to brown in color with water-soaked yellowish, red or dark brown margins. This border does not appear in bulbous iris. The spots typically enlarge and coalesce, causing the leaves to die from the tip downward. Spots can also occur on stems and flower buds.
Conditions Favoring Disease: Iris leaf spot thrives in rainy conditions in cool to warm temperatures.
How Pathogen Survives/Disperses: In moist latitudes, the fungus over-winters as mycelium in diseased leaves and other plant parts.
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Black Spot of Rose -- Diplocarpon rosae
Host: Rose
Symptoms: Black spot appears as circular spots up to 5 mm in diameter. The spots have fringed margins, a diagnostic characteristic that distinguishes this disease from other leaf spots on roses. Under favorable disease conditions, the spots enlarge, then coalesce, forming large, irregularly shaped spots. In susceptible rose varieties, it is common for the foliage to turn yellow and defoliation to occur. Excessive defoliation during the season weakens the plant, increasing its susceptibility to winter kill. Cane lesions are small, pin-head sized purple to reddish spots that are slightly raised with definite margins.
Conditions Favoring Disease: Iris leaf spot thrives in rainy conditions in cool to warm temperatures.
How Pathogen Survives/Disperses: Black spot is a serious problem in most areas where roses are grown. The fungus overwinters as mycelium in diseased leaf litter.
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Myrothecium Leaf Spot -- Myrothecium roridum
Host:begonia, dieffenbachia, gloxinia, New Guinea impatiens, philodendron, spathiphyllum, pansy
Symptoms: Symptoms of Myrothecium leaf spot for the majority of host plants include spots anywhere on leaves. The spots initially appear water-soaked and are dark brown to black in color. Irregularly shaped black sporodochia can form with a white fringe of mycelium. These spore structures appear in concentric rings within the necrotic areas on the lower surface of the leaf.
Conditions Favoring Disease: This disease occurs during conditions with temperatures between 70° and 81°F, moisture on leaves and poor greenhouse sanitation. The fungus may invade plant tissue through open wounds most easily.
How Pathogen Survives/Disperses: The fungus survives in soil or plant debris. The optimum temperature for spore germination is 82°F.
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Phyllosticta Leaf Spot -- Phyllosticta draconis
Host: Dracaena, peperomia
Symptoms: On dracaena, this disease causes yellow irregularly shaped areas to form on both surfaces of older leaves. The spots eventually turn brown with a purple margin, and pycnidia form in the lesions. Older lesions can form bright yellow halos. On peperomia, the spots begin as black specks on leaves of any age. The lesions can then enlarge to a diameter of 1 cm while rings of light and dark tissue form.
Conditions Favoring Disease: This disease occurs in areas with heavy overhead watering.
How Pathogen Survives/Disperses: The fungus overwinters in infected leaves and is spread by water splash.
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Ramularia Leaf Spot -- Ramularia cyclaminicola
Host: Cyclamen, primula
Symptoms: The older leaves of mature plants can develop diffuse yellow areas and wilt. The fungus produces abundant spores on the undersurfaces of the yellowed leaves closest to the soil surface. On the lower surface of leaves, brown spots with indefinite margins can occur that may coalesce. Opposite these spots on the leaf’s upper surface, gray patches develop and then the leaves dry and collapse. Ramularia leaf spot also occurs on the older leaves of Primula where tan- or brown-colored spots that are angular in shape appear and can be accompanied by chlorosis.
Conditions Favoring Disease: Infects older leaves of mature plants.
How Pathogen Survives/Disperses: The fungus survives in soil and can be spread by soil movement, airborne conidia and as mycelium within the integument of seed.
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Septoria Leaf Spot -- Septoria spp.
Host: Chrysanthemum, daisy, phlox, etc.
Symptoms: Septoria leaf spot initially appears as yellow or purple spots on the leaves of plants. The spots become dark brown in color as they mature. On herbaceous plants, premature withering and death of the leaves progress from the lower to the upper parts of the plant.
Conditions Favoring Disease: This disease often occurs where overhead irrigation is used. Septoria leaf spot is favored by rainy, high-humidity and slow-drying conditions.
How Pathogen Survives/Disperses: The fungus produces tiny, black-fruiting bodies (like peppergrains), visible with a hand lens, on dead areas of the leaves. The spores are then pushed out during periods of high humidity and are spread by rain or irrigation to healthy leaves. Infections can occur whenever the leaves of the plant are wet.
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Xanthomonas Leaf Spot -- Xanthomonas campestris pv. Pelargonii (most common)
Host: Geranium, zinnia, begonia, iris, poppy, poinsettia, canna, heuchera, rudbeckia, phlox, astilbe
Symptoms: Xanthomonas most often appears as small, round, brown lesions that may turn into brown wedges. Spots may be accompanied by a yellow halo or large areas of chlorosis. Wilting of individual leaves or the entire plant are signs of systemic infection.
Conditions Favoring Disease: Symptoms appear most rapidly and spread most easily in warm temperatures. Infected stock plants often do not show symptoms until cuttings are taken in the spring.
How Pathogen Survives/Disperses: The bacteria will spread by splashing water and can be picked up by root systems through subirrigation or unsanitary practices.
Photo provided courtesy of Chase Research Gardens, write up by GPN Staff.
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Chase Horticultural Research Inc. |
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