Control of survival of Ophiobolus graminis between consecutive crops of winter wheat

Under a glasshouse crop of red clover, Ophiobolus graminis (Sacc.) Sacc. in artificially colonized straws survived for longer on the soil surface than when buried, and for much longer when suspended above the soil; survival in buried straws was somewhat more prolonged in fallow soil than under clover. In field experiments with consecutive crops of winter wheat, under-sowing with red clover was not effective in reducing the incidence of take-all, possibly because of above-ground survival of the pathogen in unploughed straw. Early rotavation, however, significantly reduced disease incidence, probably because of enhanced microbial activity and competition for nitrogen in the well-aerated compost of soil and stubble.     

Carbohydrate Uptake and Metabolism of Ophiobolus graminis

The carbohydrate nutrition of Ophiobolus graminis, the cause of the take-all disease of wheat, was investigated in growth and respiration experiments. In a synthetic medium, d-mannitol was the only carbohydrate of thirteen studied which the fungus could not use for growth. However, the fungus was found to take up mannitol by an active mechanism, which was stopped by 2,4-dinitrophenol. Di- and trisaccharides were hydrolyzed extracellularly, and the monosaccharides were assimilated at different rates.     

Rhizomatous grass weeds and Ophiobolus graminis Sacc

An account is given of the infection of grass weeds by Ophiobolus granwm in a wheat crop affected by the disease. It was shown that the fungus was carried by the rhizomatous grasses, Agropyron repens and Agrostis spp. and also by Holcus lanatus. Examination of these showed the importance of Agrostis spp. in carrying over the mycelium under field conditions.     

An Enzymic Basis for Pathogenic Specificity in Ophiobolus graminis

Avenacin, the glucosidic inhibitor present in oat roots, isacted on by a specific glucosidase produced by Ophiobolus-graminisvar. avenae, which destroys biological activity, as does hydrolysiswith 0.1 N. HCl. Neither O. graminis itself nor any other fungustested produces this enzyme. It is suggested that the resistanceof oats to O. graminis and its susceptibility to var. avenaedepend on an inhibitor-inactivating enzyme complex, and thisis compared with the antibiotic-inactivating enzyme complexfound in penicillin-producing moulds and resistant penicillinase-producingbacterial strains; such a complex may be concerned in othercases of pathogenic specificity. The way in which a varietyof O. graminis pathogenic to oats may have arisen is discussed.     

Effect of Ophiobolus graminis infection on the growth of wheat and barley

Glasshouse experiments are reported, in which the development of young wheat and barley plants was examined following inoculation with Ophiobolus graminis (Sacc.) Sacc. The dry weight, leaf area, tiller number and water content of the shoot were reduced by infection. Reductions were equally severe in wheat and barley. The seminal root system of both was severely attacked and its growth retarded. Inoculated plants, however, translocated a greater proportion of their total assimilates to the root system and produced more adventitious roots than healthy plants. As a result there was an increase in both the number and proportion of healthy roots on these plants following the initial infection of their root systems. This effect was more pronounced in barley than in wheat. It is suggested that this may in part account for the reported relative tolerance of barley to take-all attack under field conditions.     

Occurrence of Ophiobolus graminis var. Avenae on wheat crops in the field

Ophiobolus graminis var. Avenae has been found on oat crops in ten counties in Scotland. It has also been shown to attack wheat crops in the same districts.     

Competitive saprophytic colonization of wheat straw

Using the ‘Cambridge method’, the effect of temperature on the colonization of wheat straw by Fusarium culmorum, Gibberella zeae, Cochliobolus sativus and C. spicifer was studied. Within the range 10–30 °C, saprophytic colonization by the four fungi was favoured by lower temperatures. This could not be attributed to the effect of temperature on the growth of the fungi in pure culture, but was probably caused by increased antagonism at higher temperatures from the general soil microflora. Contrary to previous indications, C. sativus strongly colonized straw under suitable environmental conditions.     

Soil conditions and the take-all disease of wheat: VIII. Further experiments on the survival of Ophiobolus graminis in infected wheat stubble

Assimilable nitrogen in various forms prolonged the life of Ophiobolus graminis in infected wheat straw, whether added directly to the straw or to the surrounding soil. When the infected straws were buried in washed quartz sand, 0.5 g. nitrogen per 100 g. air-dry straw was the optimum dressing for longevity of Ophiobolus. Addition of sodium phosphate did not significantly increase longevity.
Nitrogen is considered to prolong the life of Ophiobolus by enabling the mycelium to form new branch hyphae, which can explore unexhausted parts of the substrate; it is suggested that aged mycelium dies from carbohydrate starvation, through exhaustion of the zones of enzymic erosion around the hyphae. This hypothesis is supported by the extended life of the fungus in infected straws that were shaken twice weekly in 3 % dextrose solution.
Ophiobolus was found to survive longer in infected straws buried in a fallow soil than in the same soil under oats, mustard or trefoil; this finding suggests the use of catch crops as competitors with Ophiobolus for soil nitrogen.     

The effects of formalin, nabam, irrigation and nitrogen on Heterodera avenae Woll., Ophiobolus graminis Sacc. and the growth of spring wheat

In 1964, nabam (sodium ethylene bisdithiocarbamate), water and three amounts of nitrogen fertilizer were applied to spring wheat on soil treated and untreated with formalin. The experiment lasted for 3 years during which there were eight different formalin, no–formalin sequences. The nabam and irrigation treatments were discontinued when it was found they did not affect the principal pathogens present, the cereal cyst nematode Heterodera avenae and the take–all fungus Ophiobolus graminis. Formalin increased grain and straw yields in the year in which it was applied but led to increased H. avenae populations which adversely affected the succeeding crop. Formalin controlled O. graminis in the year it was applied except on land treated for the first time in 1966. H. avenae seemed to be the main check to growth until about June, and O. graminis later. At the end of the experiment, total grain yields and nematode soil populations were greatest in the plots treated with formalin each year and least in those never treated with formalin. Yield loss from either O. graminis or H. avenae alone could not be assessed because formalin usually controlled both during the season in which it was applied and both were present in untreated plots. However, in 1965, some comparisons of the effects of each pathogen were possible when one occurred in the presence of differing amounts of the other. A doubling of total grain yield over 3 years was accompanied by an eightfold increase in H. avenae in sequences of continuous formalin or formalin 1964 and 1965, whereas yield increases caused by extra nitrogen were not matched by such a big increase in H. avenae. This suggests that formalin might be affecting H. avenae through factors other than increased plant size and vigour, which in themselves would tend to encourage larger nematode populations. In the absence of formalin, H. avenae soil populations either fell or failed to increase.     

Methoxyhydroquinone, a Growth Inhibitor of Ophiobolus graminis, in Leaves of Oat Seedlings

A methanol extract of leaves of oat seedlings grown in sand cultures in the dark contained a compound which inhibited the growth of Ophiobolus graminis. The inhibitory factor was isolated and proved to be present in the plant as methoxyhydroquinone glucoside. The glucoside was readily hydrolysed to the corresponding aglucone. The methoxyhydroquinone, or possibly its oxydation product, methoxy-P-benzoquinone, was inhibitory to both Ophiobolus graminis var. graminis and Ophiobolus graminis var. avenae, whereas Fusarmm oxysporum var. lycopcrsici was not affected. Synthetic methoxyhydroquinone at 80 mg/l gave a 100% inhibition of Ophiobolus graminis var. graminis. After being exposed to 80 mg/l of the inhibitor for 24 h the mycelium was unable to initiate growth when transferred to a fresh nutrient solution. Only extracts from young leaves showed inhibitory activity, extracts from mature leaves giving no inhibition. The hydroquinone, or its glucoside, was not detected in roots of young seedlings, where avenacin was the only antifungal compound present.     

A scanning electron microscope study of interactions between micro-organisms andGaeumannomyces graminis (Syn.Ophiobolus graminis) on wheat roots

Roots of wheat grown in unsterilized sand inoculated withGaeumannomyces graminis (Sacc.) von Arx and Olivier were examined by scanning electron microscopy. Healthy roots had a mucilaginous covering and were sparsely colonized by bacteria, but asG. graminis colonized the roots the mucilage disappeared and the numbers of bacteria on the surface increased. Lysis of the hyphae occurred, apparently caused by bacteria that colonized the hyphae. Inoculation of wheat in axenic culture with a strain ofPseudomonas fluorescens that was antagonistic toG. graminis in agar gave some protection against the pathogen; lysis of hyphae was observed where protection occurred.