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III. FUNGI AS PARASITES                                                TOP

B. Destructive Fungal Parasites

1. Fungal Diseases of Plants


Fungi are parasitic on almost all groups of eukaryotic organisms, ranging from cellular amoebae, protozoa, and algae to larger plant groups such as liverworts, mosses, ferns, and seed plants, and animal groups from the smallest inconspicuous ones to the larger animals. The most intelligent animal, man, has his/her share of fungal parasites. Parasitic fungi are best known through their extensive damage to plants, especially cultivated plants.

1. Fungal Diseases of Plants

Plant Pathology is the study of (1) the living entities and the environmental conditions that cause disease in plants; (2) the mechanisms by which these factors produce disease in plants; (3) the interactions between the disease causing agents and the diseased plant; and (4) the methods of preventing or controlling disease and alleviating the damage it causes (Agrios, 1997. Plant Pathology (4th Ed.), Academic Press, New York, N.Y., 635 pp). While plant diseases may be caused by environmental factors, viruses, mycoplasma, bacteria, nematodes, a few protozoa, and parasitic higher plants, by far the majority (more than 3/4) of plant diseases are caused by fungi. All of the major groups of fungi we studied in the initial sessions of this course have species that are plant parasitic.

a. Diseases caused by Flagellated Fungi and Fungal-like Protozoa: 

Plasmodiophoromycetes  (currently in the Kingdom Protoctista): This class contains what many interpret as entophytic slime molds, because they produce amoeboid cells and plasmodia within the cells of their host. They produce resistant cells usually in packets (sori) within host cells during the culmination of infection. Such spores may remain dormant in the soil for years, or until a susceptible host is present, at which time they are stimulated to release zoospores that will infect new hosts. Their zoospores differ from fungal zoospores in having two whiplash flagella. The most important of parasites are Plasmodiophora brassicae that causes club root of cabbage and related plants, and Spongospora subterranean, the cause of powdery scab of potatoes. This group is usually studied by mycologists and plant pathologists.

Chytridiomycota: Several different groups of chytrids parasitize plants. Olpidium spp. infect pollen, algae, other fungi, and several different groups of higher plants. Physoderma maydis causes brown spot or streak of corn leaves and species of Synchytrium cause wart of potatoes. In cool, moist climates, chytrids can cause extensive damage to crop plants.

Oomycetes  (currently in the Kingdom Stamenopila): There are four orders of Oomycetes, all characterized by producing heterokont zoospores, (with one whiplash and one tinsel flagellum) asexually and oospores sexually. The most economically important group of Oomycetes is the Peronosporales that contain the late blight of potato fungus Phytophthora infestans (Fig. 12-34) and relatives such as Peronospora, Bremia, Plasmopara and others that cause “downy mildews”, the “damping off” fungi, Pythium spp., and the white rust fungi, Albugo spp. 

Fig. 12-34. Zoosporangia of Phytophthora infestans.

The impact of the late blight of potatoes (Fig. 12-35) on human populations will be discussed in a later lecture. 

Fig. 12-35. Symptoms of late blight of potato.

Worldwide, species of Phytophthora ranks high among our most destructive plant pathogens. Downy mildews, so named because of their fuzzy growth on leaves, are especially a problem on cucumbers, cantaloupes, and similar plants (Fig. 12-36). Their downiness comes from hundreds of highly branched sporangiophores (Fig. 12-37).  

Fig. 12-36. Downy mildew of cucumber.

Fig. 12-37.  Sporangiophores and spores of Psuedoperonospora. 

b. Diseases caused by nonflagellated Fungi: 

(i) Zygomycota: The most important order of Zygomycetes that cause diseases of plants and decay of plant products is the Mucorales. Members of the bread mold genus Rhizopus causes soft rots of vegetables and fruits. Species of a related fungus, Choanephora, causes blossom blight and decay of squash and similar vegetables.

(ii) Ascomycota:  This is one of the largest and most complex groups of fungi. If you will recall from an earlier lecture, there are various groups of Ascomycetes recognized by the way they produce their asci.

Archiascomycetes & Hemiascomycetes: Yeasts such as Saccharomyces and others are very important in the spoilage of grains, fruits, and vegetables.  Species of Taphrina causes swelling and distortion of leaves, flowers, and fruit of a number of plants. An important one is Taphrina deformans that causes peach leaf curl. Others cause oak leaf blister and similar foliar diseases. Nematospora causes seed decay and root rot on plants such as cotton.

Plectomycetes: The most important plant parasitic genus in the Plectomycetes is Ophiostoma (Ceratocystis).  O. ulmi is the cause of the Dutch elm disease (Fig. 12-38) which has basically eliminating the American Elm from North America. 

Fig. 12-38. Elms showing early symptoms of Dutch elm disease.

Other species cause oak wilt, blue stain of lumber, and a variety of other problems. These fungi are difficult to control because they have a symbiotic relationship with and are disseminated by bark beetles (Fig. 12-39).

Fig. 12-39. Morphological features of Ophiostoma (top), insect galleries (middle), and late stages of Dutch elm disease (bottom).

 Species of Ophiostoma are especially adapted for insect transmission because their ascomata mature and exude ascospores in a sticky droplet at the time bark beetles emerge and fly to new trees. As the insects crawl over the ascomatal beaks, they pick up spores and take them along to where the beetle will set up “housekeeping” again (Fig. 12-40). In new galleries Ophiostoma spores will germinate and form a fungal garden that will provide food for young beetle larvae. From this mycelium, new ascomata will develop and the cycle is repeated .  

Fig. 12-40. Perithecial beaks of Ophiostoma with viscid spores droplets.

In recent years the bark beetles have marched across the range of American elms, eliminating the trees from our landscape. Spore transportation is made easy because beetles have special compartments called mycangia (Fig. 12-41) that harbor spores for a very long time.

Fig. 12-41. Special cavities, mycangia, in the beetle vector that are instrumental in disseminating spores of the fungus.

Pyrenomycetes: These are the perithecial Ascomycetes that for the most part form their asci in flask-shaped ascomata, but occasionally in cleistothecia. An important cleistothecial group is the Erysiphales, or powdery mildews, that grow on the epidermis of leaves, forming asexual spores in abundance, thus, powdery mildew (Fig. 12-42), and later forming the sexual stage when cleistothecia appear.  

Fig. 12-42. Powdery mildew on the leaf of an ornamental.

There are 7 common genera of powdery mildews in North America. The genus Erysiphe is common on grasses, Phyllactinia on oaks and other trees, and Uncinula on grapes and other shrubs. Powdery mildew cleistothecia all have appendages that aid in dissemination (Fig. 12-43).  

Fig. 12-43. A high magnification of cleistothecia of  a species of Mycrosphaera.

The Xylariales usually form dark-colored perithecia in or on parasitized plants. Some important parasitic genera include Eutypha and Valsa submerged in twigs of numerous plants, Hypoxylon and Xylaria that form large stromatic ascomata on plants. Although thought to be saprobes on wood, certain species of Xylaria, like X. polymorpha, the dead man’s fingers, have been shown to cause cankers. The Diaporthales form reddish or brown ascal stromata that are normally embedded in plant tissues. Important plant pathogens include species of Diaporthe with their conidial state Phomopsis that causes diseases on soybeans, peppers, tomatoes, and a wide variety of agronomic plants. Perhaps the most famous member of this group is Cryphonectria parasitica (=Endothia of earlier literature), the cause of chestnut blight (Fig. 12-44)

Fig. 12-44. Healthy chestnut trees before the blight.

This fungus entered the US around 1904 when seedlings of Chinese and Japanese chestnuts were brought into New England by foresters. American chestnut trees suddenly started dying in the Bronx Zoo in New York in 1904. Later, Dr. W.A.Murrill (Fig. 12-45), who spent his last 30 years in Gainesville, found that the disease was caused by C. parasitica.

Fig. 12-45. A chestnut tree killed by  Cryphonectria parasitica.

 This fungus soon spread onto the American chestnut which was much more susceptible to the fungus than the European chestnut. As a result, by 1935 the fungus has spread throughout the range of the American chestnut, eliminating this most abundant lumber species from its natural habitat (Fig. 12-46)


Fig. 12-46. Chestnut trees that have succumbed to the blight.

Chestnut blight reached the Carolinas in the 1920s.  The chestnut has a remarkable power of survival and each year the root system of many plants will send forth vegetative shoots that will survive a few years before the fungus will again cause dieback. These vegetative shoots in some areas have been able to live progressively longer, indicating that they may be acquiring some type of natural resistance.  We discussed biocontrol of chestnut blight earlier. The fungus has an asexual state, Cytospora, and a sexual stage, Cryphonectria. The Cytospora stage forms large masses of spores whenever it ruptures through the bark of younger twins, making the disease difficult to control because the sticky spore masses will attach to insects, birds, and small animals and be carried to healthy trees (Fig. 12-47)

Fig. 12-47. The Cytospora stage of the chestnut blight fungus showing numerous mucilaginous coils of spores.

Older infected areas of the chestnut develop perithecia that enable the fungus to overwinter. The University of Florida was fortunate that Dr. W.A. Murrill, who discovered the cause of chestnut blight, resided in Gainesville for 28 years and while here described almost a thousand new species of fungi (Fig. 12-48).

Fig. 12-48. Dr. W. A. Murrill on the campus of the University of Florida in 1957.

The Hypocreales are perithecial Ascomycetes recognized by their fleshy and usually brightly colored perithecia. Most of them have a phialidic conidial state. Several are important plant parasites, including species of Nectria and their anamorphs of Fusarium, Verticillium, and Gliocladium. They cause wilts, cankers, root rots, and a variety of plant problems. The Clavicipitales are important plant parasites, especially on grasses in which they not only cause diseases in some but become endophytes (symbionts) and provide protective qualities to others. Claviceps purpurea occurs on wheat, rye, barley, and a number of cereal grasses. While their overall damage to grasses is not extensive, the sclerotia (ergots) are of great concern because of their toxic effects on man and other animals (ergotism). These are discussed in two other chapters on endophytes and mycotoxins.

Discomycetes are the cup-fungi and their allies. Important plant pathogens in this group include Rhytisma, the cause of tar spot of maple, Sclerotinia which causes various stromatic rots of vegetables, and Monilinia, the cause of brown rot of peaches and similar fruits. We discussed brown rot of stone fruits earlier in the lectures on biocontrol of plant diseases. Brown rot gets its name from the fact that infected fruit eventually turn brown (Fig. 12-49) and become mummified and often cling to the tree (Fig. 12-50)

Fig. 12-49. The brown rot of peach caused by Monilinia fructicola.

Fig. 12-50. The mummy stage of brown rot of peach.

Fallen mummies will overwinter on the soil and when warm spring rains and susceptible blossoms and leaves emerge, apothecia will appear (Fig. 12-51) and liberate spores that will reinfect. 

Fig. 12-51. Overwintering peach mummies give rise to apothecia.

Species of Lophiodermium and its relatives cause needle cast of a number of cone-bearing plants.

Loculoascomycetes: This is a large groups of stromatic Ascomycetes in which the bitunicate asci are borne within stroma and lyse a cavity; thus the name  locule. From one to six orders have been recognized in this group, all have plant parasitic members. Among the Myriangiales there is citrus scab caused by species of Elsinoe.  In the Dothideales there are species of Capnodium that cause sooty molds of plants, Mycosphaerella that cause leaf spots of plants, and one that causes  greasy spot of citrus. Among the Pleosporales, Venturia inaequalis, the cause of apple scab (Fig. 12-52), has small stroma with individual pseudothecia within (Fig. 12-53). 

Fig. 12-52. Symptoms of apple scab.


Fig. 12-53. Section through a psuedothecium of Venturia inaequalis.

Species of Gaeumannomyces cause takeall disease of wheat. 

  (iii) Deuteromycetes: This is an artificial group of fungi that is made up of the conidial (asexual) states of various fungi, but largely Ascomycetes. Species of Alternaria, Bipolaris, Botrytis, Cercospora, Diplodia, Dreschlera, Exerohilum, Fusarium, Phoma, Phomopsis, Rhizoctonia, and Verticillium are among the most common groups that cause molds, blights, cankers, leaf spots, root rots and other maladies.

(iv) Basidiomycetes:  Among plant diseases caused by Basidiomycetes we find the rusts, smuts, felt fungi, root rots, heart rots, and thread-blights.

The Uredinales represent the rust fungi (Fig. 12-54)

Fig. 12-54. Different stages of the wheat stem rust.

They cause serious disease on many of our most important agronomic plants, the grasses & cereals. Rust fungi have been very difficult to control because many of them are  heteroecious, i.e. they require two hosts on which to complete their life cycle. Many of the alternate hosts are herbaceous  or forest species that make it all but impossible to locate and control; thus, a fresh supply of inoculum is always availabe for the host that we prize. Some of the most important rust fungi include Puccinia graminis with many varieties that attach wheat, oats, rye, barley etc. One of the tactics used in the control of cereal rusts is the breeding of resistant varieties of the host. Unfortunately, the fungus with its shorter life cycle, large number of replicating cycles, and mating on a separate host, has been able to quickly make genetic changes necessary to infect new cereal varieties as they are introduced. Other important rusts include Puccinia arachides on peanuts, species of Cronartium that causes rusts of pines, with an inconspicuous stage on many of our oak species; Phragmidium, the rust of roses, and Melampsora on flax , beans and other hosts, and species of Gymnosporangium that causes cedars-apple rust in which the fungus alternates between cedar (Fig. 12-55) and apple trees (Fig. 12-56).  


Fig. 12-55. The telial stage of cedar-apple rust, Gymnosprangium  juniperi-virginiana.

Fig. 12-56. The aecial stage of cedar-apple rust on apples.

The Ustilaginales encompass the smut fungiTheir spores are formed inside of blister like pustules on leaves, but more commonly on inflorescences of plants. When spores mature in these blister-like pustules they give a smutty appearance (Fig. 12-57).  

Fig. 12-57. What appears to be smoke is a severe epidemic of bunt smut of wheat caused by Tilletia caries

Species of Ustilago are common on corn (Fig. 12-58), wheat, rye and barley. 

Fig. 12-58. Common corn smut caused by Ustilago maydis. Incidentally, highly prized for food in Latin America.

Tilletia causes bunt and stinking smut of wheat. Karnal bunt of wheat is currently of great concern to U.S. wheat growers because many countries have quarantines against wheat with this smut. Recently, this smut fungus appears to have moved from Mexico into some wheat growing areas of New Mexico, Arizona, and Southern California. Will wheat from the U.S. be on quarantine in other countries, or can the fungus be eliminated? Most of you have probably seen the smut on onion, the black stuff between the onion bulb scales, caused by Urocystis.

The Exobasidiales are a small group of mainly leaf parasites that occur mostly on Ericaceous plants, i.e. the blueberry family. They cause galls very similar to the galls that are formed by Taphrina in the peach leaf curl. Often, not only leaves of the host are infected with Exobasidium but also the inflorescences which become large, lime green, and succulent.

While most of the Aphyllophorales (bracket fungi) are wood rotting species, several groups become parasitic and cause plant diseases. They include Athelia that causes root and stem rots; the most important of which is A. rolfsii (Fig. 12-59), named after a famous plant pathologist from the University of Florida, P.A. Rolfs. 

Fig. 12-59. Stem rot of peanuts caused by Athelia rolfsii.

Species of Ceratobasidium also cause  thread blight. Heterobasidium, that we examined in the biocontrol section, causes root rots in living trees, and Thanatephorus causes stem and root rots of a wide variety of plants. The mushroom order Agaricales is largely mycorrhizal or saprobic, but a few are clearly parasitic. This includes species of Armillaria (Fig. 12-60) that causes root rots on a number of trees and ornamental species.  

Fig. 12-60. Mushroom root rot is caused by Armillaria tabescens.