THE BASICS OF PLANT DISEASES

by Joelle Steele

Foliage diseases, like the plants they infect, have common names and Latin names. The common names, such as "root rot" or "leaf spot" are very vague as they may refer to several different kinds of diseases. For example, there are several leaf spot diseases such as Xanthomonas, Leptosphaeria, Fusarium, and Phyllosticta, just to name a few. And, each of those may have variations as is the case with Phyllosticta which is more specifically named P. maculicola, P. dracaenae, P. concava, etc.

In order to correctly treat a disease, it is important to be able to diagnose it accurately. Knowing the pathogen (bacteria, fungus, or virus) will enable you to select the correct means of control (dust, fumigant, etc.). Also, most plants have certain diseases to which they are most susceptible which does help you narrow the choices somewhat.

Alternaria actinophylla (Alternaria blight). Frequently affects scheffleras with brown or black lesions with yellow haloes. Abscission of leaves is common. In severe infections stems become infected and black fungal spores are apparent.

Botrytis cinerea (Gray mold). Most often found on ficus. Consists of brown leaf spots with concentric rings. Necrosis between leaf sheathes and new leaves may develop.

Cercospora (Leaf spot). Most often found on scheffleras and cordylines, symptoms include minute, edema-like swellings on the underside of leaves which later turn yellow and abscise. Lesions may turn reddish brown with a chlorotic border.

Colletotrichum omnivorum (Leaf spot). Most often found on aspidistras. Characterized by white spots and brownish leaf and petiole margins.

Dactylaria humicola (Leaf spot). In philodendrons, the newer leaves have tiny watery spots on the undersurface. As the leaves age, the lesions turn yellow with a brown watery center. The centers collapse leaving dents that look a bit like a thrips infestation.

Dasheen mosaic virus. Affects members of the Araceae family. In aglaonemas and dieffenbachias, symptoms are mosaic patterns on the leaves, stunting of growth and disfiguration of the plant. Usually looks bad without the plant actually dying. In cordatums, the disease is apparent on new leaves and is characterized by chlorotic bands along the veins with some distortion and stunting of growth.

Erwinia chrysanthemi (Erwinia blight). Most often found on aglaonemas, syngoniums, and sansevierias. In aglaonemas, symptoms include spots surrounded by yellow haloes, and with internal infection, leaves may wilt, yellow, and collapse. In sansevierias, brown or black root and crown rot, leaf spots, wilting and collapsing of plant structures, and brownish-black leaf discoloration. In syngoniums, oval and irregular black leaf spots with necrotic haloes. With internal infection leaves become mushy and collapse or abscise. In dieffenbachias, E. dieffenbachiae causes brown, soft lesions on the stems and brown, soft spots on the leaves. In pothos, E. carotovora ("Rapid Decay") begins as a soft, wet, gray green area which becomes larger and turns black. If it dries out, the leaf lesions will turn dark with yellow margins. In very short time, the infected tissue collapses. Erwinia-infected plants should be destroyed.

Exosporium palmivorum (Leaf spot). Found on palms, the spots are small, circular, yellow, and clear. Sometimes they blend together and form grayish brown lesions which destroy the leaves.

Fusarium moniliforme (Leaf spot). Found most often in dracaenas, palms, and sansevieria. Leaf spots are round, raised, rust colored lesions with yellow haloes. Off-white spores appear at the growing point where the apex may rot.

Gliocladium. In palms, the most common symptom is the death of the oldest leaves which have necrotic streaks in the leaflets which turn yellow or brown. Rose colored spores are present.

Glomerella cincta (Leaf spot). In dracaenas and marantas the leaves begin to discolor from the tips towards the base.

Glomerella cingulata (Anthracnose). Most likely found on ficus. In initial stages tips turn yellow, then brown, then dark brown. Sometimes the leaf margin is affected. In more advanced stages pinkish pustules develop along the leaf veins and black spore sacs appear on the leaves.

Phyllosticta maculicola (Leaf spot). In dracaenas and cordylines this disease is characterized by brown leaf spots with yellow haloes. With P. dracaenae, the spots are irregular, brown with purple borders and yellow haloes, and appear on the lower surfaces of older foliage.

Phytophthora (Leaf spot). This disease affects aglaonemas, cordylines, dieffenbachias, philodendrons, and African violets, among other species. It is related to Pythium, causes mushy, watery, leaf spots which turn rust colored with yellow haloes when dry. Severe infections, result in complete plant collapse.

Pseudomonas alboprecipitans (Bacterial blight). Most palms are susceptible, caryotas in particular. Symptoms are wet areas around long brown lesions with necrotic centers, running parallel to the leaf vein. New leaves are most severely affected. Remove the infected foliage and keep it dry to control the disease.

Pythium. Common to many foliage plants, this water mold is encouraged by wet conditions, most often due to overwatering in a cool environment. Leaves yellow from the base up and wilt. The real give away is in the roots which collapse and darken with the outer layer of the root rotting away completely leaving only the thread-like stele in place.

Rhizoctonia. Effects aglaonemas, chamaedoreas, cordatums, dieffenbachias, pothos, hoya, marantas, philodendrons, pothos, and scheffleras, to name a few. The entire plant and soil surface may be effected with rusty-colored minute filaments (the fungal body).

Sclerotium rolfsii (Southern blight). This disease effects scheffleras, pothos, philodendrons, dieffenbachias, cordatums, and syngoniums, among others. There is usually a white fungus, complete with tan spore-like objects, on the soil surface and the stem at the soil line. The stem rots and the plant collapses.

Sphaerotheca humili (Powdery mildew). Appearing in many plants, this disease is characterized by grayish or white mildew on the leaves. As the infected parts dry out the leaves die. Control is extremely difficult. Infected plants should be removed as quickly as possible to avoid further contamination.

Xanthomonas dieffenbachiae (Leaf spot). In dieffenbachias, symptoms include yellow or gold spots (that later turn rusty in color) with clear centers on the top of the leaves and a silverish waxy substance on the under surface. In severe infections leaves yellow, wilt, and die. In cordatums, small wet spots turn yellow, later brown, and leaf margins turn yellow, later rust-colored, starting at the tip, and abscise. In syngoniums, X. vitians produces watery lesions on the leaf margin and tip which turn yellow, then brown with a yellow area surrounding the necrotic region.

As with most plant problems, pathogenic foliage diseases are most often originated in greenhouse environments. They are uncommon in the indoor environment which, in most cases, lacks sufficient humidity to encourage their growth and dispersal. For this reason, you should be particularly careful in examining new purchases, and all technicians should be cautioned to exercise proper maintenance hygiene.

To discourage these diseases in your greenhouse or in atrium environments, take care to use clean soil, provide adequate drainage, space plants to eliminate crowding, remove dead leaves or any dead tissue from the plant (or container) immediately, and avoid overwatering or splashing water on the leaves or stems.

To control a disease, cut back on the water and use an appropriate fungicide (or bacteriacide) mixed and applied in accordance with the directions on the label. You will probably have to apply the fungicide twice, approximately 90 or so days apart.