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.     

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.     

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.     

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.     

Effects of nitrogen and glucose on saprophytic survival of Ophiobolus graminis in buried straw

Calcium nitrate prolonged the saprophytic survival of Ophiobolus graminis (Sacc.) Sacc. in artificially colonized straws buried in soil in the laboratory, whether supplied to the soil (at 12·5 or 100 mg nitrogen/100 g soil) or to the straws before colonization (at 0·5 or 1·0 g nitrogen/100 g straw). Glucose (at 2·5 g/100 g soil, and at 10 g/100 g straw) shortened survival. When straws colonized in the presence of 0·5 g nitrogen/100 g straw were buried in soil supplied with 14·1 mg nitrogen/100 g soil, the level of soluble soil nitrogen reached equilibrium at 2–4mg/100g soil; this allowed rapid straw decomposition and, although the added soil nitrogen prolonged survival in straws that remained undecomposed, it also accelerated substrate exhaustion. Addition of 100 mg nitrogen/100 g soil was supra-optimal for survival: although some nitrogen was necessary for maximum survival, the equilibrium concentration of soluble nitrogen (24–56 mg/100 g soil) was high enough in this case to have an inhibitory effect in addition.     

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.     

Effects of break crops,and of wheat volunteers growing in break crops or in set‐aside or conservation covers,all following crops of winter wheat,on the development of take‐all (Gaeumannomyces graminis var. tritici) in succeeding crops of winter wheat

Experiments on the Rothamsted and Woburn Experimental Farms studied the effects on take‐all of different break crops and of set‐aside/conservation covers that interrupted sequences of winter wheat. There was no evidence for different effects on take‐all of the break crops per se but the presence of volunteers, in crops of oilseed rape, increased the amounts of take‐all in the following wheat. Severity of take‐all was closely related to the numbers of volunteers in the preceding break crops and covers, and was affected by the date of their destruction. Early destruction of set‐aside/conservation covers was usually effective in preventing damaging take‐all in the following wheat except, sometimes, when populations of volunteers were very large. The experiments were not designed to test the effects of sowing dates but different amounts of take‐all in the first wheats after breaks or covers apparently affected the severity of take‐all in the following (second) wheats only where the latter were relatively late sown. In earlier‐sown second wheats, take‐all was consistently severe and unrelated to the severity of the disease in the preceding (first) wheats. Results from two very simple experiments suggested that substituting set‐aside/conservation covers for winter wheat, for 1 year only, did not seriously interfere with the development of take‐all disease or with the development or maintenance of take‐all decline (TAD). With further research, it might be possible for growers wishing to exploit TAD to incorporate set‐aside/conservation covers into their cropping strategies, and especially to avoid the worst effects of the disease on grain yield during the early stages of epidemics.     

Grain yield and incidence of take-all (Ophiobolus graminis Sacc.) in wheat grown in different crop sequences

The yield of wheat and the incidence of take-all were measured in crops grown in six different 4-year sequences, repeated in 3 successive years. The first crop of winter wheat grown after oats or beans yielded 13–23 cwt/acre (1632–2887 kg/ha) more grain than wheat after wheat or barley. Spring wheat after oats yielded 2–5 cwt/acre (250–625 kg/ha) more than spring wheat after wheat. The smaller yields of wheat after wheat or barley were caused mostly by greater prevalence of take-all. Regression analysis indicates that each 1 % increase in straws with take-all decreased yield of winter wheat by 0·6%. Take-all was more prevalent in the second and third successive wheat crops after oats than in the fourth crop.     

Effect of Ophiobolus graminis infection on the assimilation and distribution of 14C in wheat

The uptake of ¹⁴C and movement of ¹⁴C-labelled assimilates in wheat plants inoculated with Ophiobolus graminis was examined following exposure of the second youngest leaf to ¹⁴CO₂. Autoradiographs of plants with infected seminal roots showed that assimilates were not translocated past the sites of root infection but accumulated in the undamaged portions of infected root systems, in particular the developing crown roots. There was no evidence that assimilates accumulated in the vicinity of O. graminis lesions. The net assimilation of ¹⁴CO₂ by wheat plants over a 5 h feeding period was not significantly affected by O. graminis infection. However, infection reduced the amount of ¹⁴C lost through respiration. Infection delayed the transfer of labelled assimilates from the fed leaf to the remainder of the plant but increased the proportion translocated to the roots. The latter effect was not apparent when infected plants were continuously irrigated during, and for 20 h following, the feeding period.     

Analysis of spatial and yearly variation in winter survival of winter wheat

 Four years of winter survival data for winter wheat (Triticum aestivum L.) were collected on a loam soil located on the Central Experimental Farm at Ottawa, Ontario (45° 23′N, 75° 43′W). The site was low-lying and subject to frequent winter flooding and ice sheet formation. It appeared level although there was microtopographic variation with a range in elevation of approximately 0.15 m. The objective of the study was to gain insight into factors which might affect winter survival. Two varieties, a soft white and a hard red winter wheat, were planted in September. Crop establishment was measured in late fall and the percentage survival was measured in April of the following year. We assumed the large spatial differences in survival were not totally random, but rather were affected by spatial variation in environmental factors such as snow and ice depth, soil moisture and temperature. Hourly measurements of soil temperature at a depth of 0.05 m were recorded throughout the fall, winter and spring. Fall and spring soil moisture at the same depth were measured on the plot, as well as snow and ice depth at selected times throughout the winter. Measurements were taken on a grid covering the plot to help explain spatial variation in survival. In addition, detailed measurements of elevation were taken on a grid. Meteorological data were available from the nearby weather site. While soil temperatures were never low enough to kill plants according to the CERES model, the spatial variation in winter survival was associated with differences in elevation and the resulting surface drainage patterns. Received: 23 March 1998/Accepted: 21 October 1998     

The relationship between populations of stem-boring Dipterous larvae in leys and damage to subsequent winter wheat crops

During September of each year from 1978-82, dipterous stem-boring larval populations were assessed in 28–35 grass fields in northern England before they were ploughed and sown with winter wheat. Larvae were mainly Oscinella spp. and their populations ranged from <60–3100/m² (mean 300/m²). Subsequent damage to winter wheat assessed during December of each year from 1978-81 was positively related to the larval population in the grass. Regressions of plant damage on larval numbers indicated that 500, 1000 and 2000 larvae/m² resulted in respectively 10, 19 and 36% damaged wheat plants. Mean populations were greatest (558/m²) in 2-yr-old leys containing Lolium multijlorum. Three-yr-old leys had a mean of 319/m² and leys of 4 yr and older had 168 larvae/m². Soil type and management of leys had little consistent effect on larval populations. Populations tended, however, to be larger on silty or silty-clay loams than sandy loams and where leys were grazed in late summer after earlier silage cuts compared with those continuously grazed.     

Effect of previous crops on the incidence of eyespot on winter wheat

Surveys of winter wheat from 1939 to 1946 show that eyespot (Cercosporella herpotrichoides Fron.) occurs throughout Britain and that its incidence depends largely on previous cropping and on weather. Examination of 551 crops on land whose cropping for the previous 4 years was known showed that the incidence rose steadily with increasing numbers of preceding wheat and barley crops: where neither crop had been taken for 4 years the proportion of crops with more than 70% infected straws was 2%, rising to 45% where three or four such crops had been taken and the average straws infected rose from 6 to 55%.
The percentage infection to be expected in various groups of crops was calculated from previous cropping; it was compared with the actual infection and so used to assess the importance of other factors in determining the incidence of eyespot. High spring rainfall, early sowing and a dense plant increased incidence and low spring rainfall, late sowing and a thin plant reduced it.
Eyespot was not usually severe on newly ploughed grassland until the third or fourth crop of wheat, but under very wet conditions it was sometimes severe in the second crop.
Oats is much less susceptible than wheat or barley, but some crops were found with a third of their straws infected.
A brief survey of winter wheat in Holland suggested possible causes for the rise and fall of eyespot in recent years and for its present lower incidence there as compared with East Anglia.     

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.