In contrast to the extensive list of insects that are affected by endophytes, the recorded number of plant diseases that are influenced by the presence of endophytes in cool-season grasses is small. Endophytes and their effects on plant diseases have not been as extensively investigated as those of insects, and this may be explained partly by the lack of success in finding many diseases that are affected by endophytes.
The first record of an endophyte affecting a plant disease was that by Shimanuki (1987), who showed that timothy (Phleum pratense L.) plants infected with the choke fungus, Epichloë typhina (Pers.) Tul. & C. Tul., were resistant to the fungus Cladosporium phlei (Gregory) de Vries. This pathogen causes a serious leafspot on timothy, but one cannot advocate using E. typhina-infected plants to control the disease because the endophyte prevents seed production. When flower heads first appear, mycelia of E. typhina emerge from within the flag leaf sheath of timothy plants and surround the developing flower heads in their stromata, thus preventing the grass from flowering. Koshino et al. (1989) described compounds produced in the mycelial-choked heads of timothy as toxic to some fungi, including sesquiterpenes, chokols, hydroxyl-unsaturated fats, phenolic glycerides, and an aromatic sterol.
Studies on turfgrass plots at Rutgers University in New Jersey have shown that cultivars and selections of turf fescues infected with Epichloë endophytes (see Chapter 26), when compared with E- cultivars, had significantly enhanced resistance to dollar spot disease caused by the fungus Sclerotinia homeocarpa (Clarke et al., 1994). Dollar spot is an important disease of fine turfgrass and so the use of endophytes in turfgrass fescues would be beneficial, provided strains of Epichloë could be used that did not interfere with seed production. Researchers at Rutgers also have shown that endophytes in fine fescues increase resistance to red thread disease caused by Laetisaria fuciformis (Bonos et al., 2005).
There is evidence to suggest that soil fertility can affect the degree of infection with choke fungus of E+ fine fescues. Sun et al. (1990) found that the rates of N fertilizer used in commercial seed production largely eliminated choke symptoms in fescues grown in old, N-deficient plant nurseries. Hence, it is possible to reduce the amount of seedhead choking in seed crops through simple N management.
The possibility of controlling plant diseases by infection with the more desirable Neotyphodium endophytes, which do not affect or inhibit flowering, was encouraged by the in vitro experiments of White and Cole (1985) and Siegel and Latch (1991). They showed that isolates of Neotyphodium endophytes grown on agar plates inhibited the colony growth of a number of fungi pathogenic to grasses. Siegel and Latch (1991) also found that the antifungal activity differed among strains of endophyte species; it is possible that this may also occur in field situations. They grew single colonies of different strains of endophyte species on agar plates and approximately a week later these plates were inoculated with fungi pathogenic to grasses. The colony growth of many of these pathogenic fungi was not inhibited by the endophyte colonies, but a few fungi, such as Rhizoctonia solani Kühn, R. zeae, Bipolaris sorokiniana, and Colletotrichum graminicola (Ces.) G.W. Wils., were restricted in growth.
Although some endophytes produced antifungal chemicals in culture, they may not be able to produce the compounds in sufficient quantities in the plant to protect the host from fungal diseases. In fact it is often found with other microorganisms that there is a poor correlation between antibiosis in culture and disease suppression in the field (Cook and Baker, 1983). A technique to investigate the effect that Neotyphodium endophytes growing in plant tissue had on different grass pathogenic fungi was devised by Christensen (1996). Small pieces of grass pseudostems containing Neotyphodium were surface-sterilized and placed into wells of 96-well ELISA plates along with sterile water. They were left overnight and then a small plug of the fungal pathogen was placed in the well. The growth of the hyphae on the fungal plug was measured during the following 24 h. It became apparent that antifungal substances were produced by many strains of Neotyphodium endophytes growing in tall fescue and perennial ryegrass because the growth of the grass pathogens Drechslera erythrospila (Drechsler) Shoemaker and Rhizoctonia zeae was restricted.
There have been several reports of diseases being affected by the presence of Neotyphodium endophytes in grasses. Wheatley et al. (2000) conducted a trial with sheep grazing perennial ryegrass in Australia and found that the leafspot fungus Pyrenophora semeniperda (Brittlebank & Adam) was present on leaves of E- perennial ryegrass plants but not on ryegrass infected with the endophyte N. lolii. Not only was the disease suppressed when the endophyte was present, but sheep preferred grazing E+ grass when the leaf spotting disease was present on the E- grass; they showed no preference at those times of the year when the disease was absent. This illustrates that N. lolii infection, or lack thereof, can impact higher trophic level organisms, such as ruminants.
Gwinn and Gavin (1992) grew tall fescues infected with N. coenophialum (Morgan-Jones and Gams) Glenn, Bacon, and Hanlin in a greenhouse and found that the E+ seedlings were more resistant to R. zeae than E- seedlings. However, field trials by Burpee and Bouton (1993) showed that endophyte infection gave no protection to tall fescue plants from the soil-borne pathogen R. solani.
West et al. (1989) grew tall fescue plants infected with N. coenophialum and E- plants in a greenhouse trial and observed that the endophyte seemed to reduce the severity of a crown rust (Puccinia coronata Corda) outbreak. However, stem rust infection by P. graminis subsp. graminicola Pers. of tall fescue growing in field plots in Oregon was not affected by infection with N. coenophialum (Welty et al., 1993). Crown rust on perennial ryegrass plants growing in Louisiana and infected with the endophyte N. lolii was observed to be less severe than on uninfected plants (Clay, 1989). Observations in New Zealand (Latch, unpublished data, 2001) have shown no effect of N. lolii on crown rust infection in perennial ryegrass. Within species of endophytes and fungal pathogens there are many different strains. Rusts in particular have many strains or races that differ in various regions and countries. This may account for investigators finding that plant diseases are affected by endophyte infection in some situations and not in others.
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