Effects of eyespot on the yield of winter wheat

In two experiments, on Julia and Zephyr spring barley, evidence was obtained that the development of powdery mildew in sprayed plots depended very much on their proximity to unsprayed crop. The effective period of control achieved with one spray was greater where plots were well separated from unsprayed crop than where they were surrounded by or very close to unsprayed crop. There is probably a risk that small plot experiments may not only underestimate the potential effectiveness of fungicides in agricultural practice, but also give misleading information on optimal spray timing.     

Influence of direct-drilling of winter wheat on incidence of take-all and eyespot

Winter wheat drilled directly into stubble or pasture treated with paraquat to kill the vegetation has been found to be less severely attacked by take-all (Ophiobolus graminis (Sacc.) Sacc.) and eyespot (Cercosporella herpotrichoides Fron) than wheat drilled after cultivation. The reduction of take-all is associated, not with a direct effect of the chemical, but with factors, resulting from the technique, which limit the rate of spread of the fungus in the undisturbed soil.     

Effects of water potential and temperature on the development of eyespot lesions in wheat

Growth of Pseudocercosporella herpotrichoides on potato dextrose agar at water potentials from -0.5 to -6.9 MPa was optimal at 20°C. At 12 and 20°C, six isolates of P. herpotrichoides grew more rapidly at -0.5 to -2 MPa than at -6.9 MPa. Wheat plants inoculated with P. herpotrichoides and grown in columns of soil at either 15 or 20°C developed more severe eyespot lesions under a heavy watering regime than under medium or light watering regimes. P. herpotrichoides penetrated leaf sheaths of inoculated plants grown in compost more rapidly at 10°C night/15°C day temperatures than at 5/10°C; death of leaf sheaths was also more rapid and consequently there were fewer living infected leaf sheaths at 10/15°C than at 5/10°C. Irrigating for 5 wk before harvest increased the severity of eyespot lesions in a 1983 wheat crop.     

Observations on take-all and eyespot diseases of wheat in Yorkshire

Wheat crops were surveyed in Yorkshire from 1944 to 1946 on farms where crops were reported to be unsatisfactory. Take-all ( Ophiobolus graminis (Sacc.) Sacc.) and eyespot ( Cercosporella herpotrichoides Fron.) were both found to be present; the latter is the most serious trouble on the better wheat lands.
The variation in the incidence of these diseases on selected farms during the three seasons has been compared.
The effect of rotation has been examined; both diseases were found to be encouraged by too frequent cropping with wheat or barley. The incidence of disease in wheat crops following a 1-year seeds ley was found to be influenced by the nurse crop used to undersow the seeds, and also by the time at which the ley was ploughed up. Oats were found to be preferable to either wheat or barley as a nurse crop, and in 1946 late ploughing considerably reduced the amount of eyespot disease present in the following wheat crop.
A 1-year ley is considered to be of too short a duration to ensure the disappearance from the soil of Ophiobolus graminis and Cercosporella herpotrichoides surviving from the previous crop, and a period of 2–3 years is suggested as being desirable.     

The effect of eyespot (Cercosporella herpotrichoides Fron.) on wheat and the influence of nitrogen on the disease

The effect of eyespot throughout the season on wheat receiving different amounts of nitrogen was studied in pot experiments. Plants inoculated in December showed chlorosis of outer leaves in February. Among plants with high nitrogen, eyespot killed 11%, caused straggling of 31% and whiteheads in 14% of the surviving ear-bearing straws, reduced yield of straw by 8% and of grain by 16%. The loss in straw yield was due to reduction in plant number, that of grain was about half due to reduction in number of ears and half to production of lighter grains. Among plants with low nitrogen the disease killed 23% of the plants, caused straggling of 86% and whiteheads in 18% of the surviving ear-bearing straws, and reduced yield of straw by 23% and of grain by 44%. The loss in straw yield was due to death of plants, that of grain was about two-thirds due to the production of fewer ears and one-third to that of lighter grains. In the high-nitrogen series the number of shoots at the time of maximum tillering was reduced by 29%; in the low-nitrogen series the disease caused reduction in height, a very uneven crop, delay in ear and anther emergence, and an increase in tail corn from 4% in the controls to 30% in the inoculated plants.
All inoculated plants became infected, but those receiving high nitrogen had only 49% of the ear-bearing straws with severe lesions at harvest, while those receiving low nitrogen had 86%. The larger number of tillers produced when nitrogen was applied may have enabled the less severely diseased shoots to survive and bear ears while the most severely infected died.     

Observations on the prevalence and incidence of ergot disease in Great Britain with special reference to open-flowering male-sterile cereals

Ergot (Claviceps purpurea) of grasses is prevalent throughout Britain. The disease also occurs regularly in most cereal varieties but generally is of low incidence. Incidence is much greater in open-flowering and male-sterile cereals, with the infection seeming to come, initially, from the large reservoir of inoculum amongst grasses. The production of F₁-hybrid cereal seed, which necessitates the use of male-sterile lines, could therefore be jeopardised.     

Mapping QTL for resistance to eyespot of wheat in Aegilops longissima

Eyespot is an economically important disease of wheat caused by the soilborne fungi Oculimacula yallundae and O. acuformis. These pathogens infect and colonize the stem base, which results in lodging of diseased plants and reduced grain yield. Disease resistant cultivars are the most desirable control method, but resistance genes are limited in the wheat gene pool. Some accessions of the wheat wild relative Aegilops longissima are resistant to eyespot, but nothing is known about the genetic control of resistance. A recombinant inbred line population was developed from the cross PI 542196 (R) × PI 330486 (S) to map the resistance genes and better understand resistance in Ae. longissima. A genetic linkage map of the S(l) genome was constructed with 169 wheat microsatellite markers covering 1261.3 cM in 7 groups. F(5) lines (189) were tested for reaction to O. yallundae and four QTL were detected in chromosomes 1S(l), 3S(l), 5S(l), and 7S(l). These QTL explained 44 % of the total phenotypic variation in reaction to eyespot based on GUS scores and 63 % for visual disease ratings. These results demonstrate that genetic control of O. yallundae resistance in Ae. longissima is polygenic. This is the first report of multiple QTL conferring resistance to eyespot in Ae. longissima. Markers cfd6, wmc597, wmc415, and cfd2 are tightly linked to Q.Pch.wsu-1S ( l ), Q.Pch.wsu-3S ( l ), Q.Pch.wsu-5S ( l ), and Q.Pch.wsu-7S ( l ), respectively. These markers may be useful in marker-assisted selection for transferring resistance genes to wheat to increase the effectiveness of resistance and broaden the genetic diversity of eyespot resistance.     

Effects of host social hierarchy on disease persistence

The effects of social hierarchy on population dynamics and epidemiology are examined through a model which contains a number of fundamental features of hierarchical systems, but is simple enough to allow analytical insight. In order to allow for differences in birth rates, contact rates and movement rates among different sets of individuals the population is first divided into subgroups representing levels in the hierarchy. Movement, representing dominance challenges, is allowed between any two levels, giving a completely connected network. The model includes hierarchical effects by introducing a set of dominance parameters which affect birth rates in each social level and movement rates between social levels, dependent upon their rank. Although natural hierarchies vary greatly in form, the skewing of contact patterns, introduced here through non-uniform dominance parameters, has marked effects on the spread of disease. A simple homogeneous mixing differential equation model of a disease with SI dynamics in a population subject to simple birth and death process is presented and it is shown that the hierarchical model tends to this as certain parameter regions are approached. Outside of these parameter regions correlations within the system give rise to deviations from the simple theory. A Gaussian moment closure scheme is developed which extends the homogeneous model in order to take account of correlations arising from the hierarchical structure, and it is shown that the results are in reasonable agreement with simulations across a range of parameters. This approach helps to elucidate the origin of hierarchical effects and shows that it may be straightforward to relate the correlations in the model to measurable quantities which could be used to determine the importance of hierarchical corrections. Overall, hierarchical effects decrease the levels of disease present in a given population compared to a homogeneous unstructured model, but show higher levels of disease than structured models with no hierarchy. The separation between these three models is greatest when the rate of dominance challenges is low, reducing mixing, and when the disease prevalence is low. This suggests that these effects will often need to be considered in models being used to examine the impact of control strategies where the low disease prevalence behaviour of a model is critical.     

The role of antibiotic production by a strain of Pseudomonas fluorescens in the suppression of Pseudocercosporella herpotrichoides, the causal agent of eyespot disease of cereals

Pseudomonas fluorescens strain 220 is an effective antagonist of Pseudocercosporella herpotrichoides , the eyespot pathogen of cereals. Culture filtrates of Ps. fluorescens 220 were inhibitory to spore germination and hyphal growth of P. herpotrichoides and at least two compounds with antifungal and antibacterial activity were identified in cultures grown in nutrient broth. In plant tests, both a culture broth of Ps. fluorescens 220 and a crude antibiotic extract reduced eyespot disease, whereas a mutant strain of 220 deficient in antibiotic production had no effect. Production of antibiotics would therefore appear to be a major factor in the suppression of P. herpotrichoides infection. A loss of disease control when Ps. fluorescens 220 was applied to plants in water was not due to lack of survival, as populations of a marked strain of Ps. fluorescens 220 applied to the stem base of wheat plants were similar whether applied in water or culture broth.     

Approaches to increasing the salt tolerance of wheat and other cereals

This review describes physiological mechanisms and selectable indicators of gene action, with the aim of promoting new screening methods to identify genetic variation for increasing the salt tolerance of cereal crops. Physiological mechanisms that underlie traits for salt tolerance could be used to identify new genetic sources of salt tolerance. Important mechanisms of tolerance involve Na+ exclusion from the transpiration stream, sequestration of Na+ and Cl- in the vacuoles of root and leaf cells, and other processes that promote fast growth despite the osmotic stress of the salt outside the roots. Screening methods for these traits are discussed in relation to their use in breeding, particularly with respect to wheat. Precise phenotyping is the key to finding and introducing new genes for salt tolerance into crop plants.     

Studies of fluctuations in insect populations: XI. The interrelationship of the wheat blossom midges and their host plant

The date of wheat blossom midge emergence in an insectary some distance away from the growing wheat can give as reliable an estimate of the forwardness of the crop as field observation of the date of ear emergence. The percentage grain attacked by the wheat blossom midges, which can be assessed by routine workers without any special experience of wheat, can give in early July a useful first estimate of the crop yield.