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.     

A method for inoculating wheat with Cercosporella herpotrichoides

A method for preparing inoculum of Cercosporella herpotrichoides on chemically defined substrates is described. The method is reproducible, needs less time, is less variable than using colonized cylinders of straw and allows infection by mycelium and spores above soil level.     

Strategies for control of eyespot (Pseudocercosporella herpotrichoides) in UK winter wheat and winter barley

This review considers strategies for control of eyespot (Pseudocercosporella herpotrichoides) in winter wheat and winter barley in the UK. Varietal resistance to eyespot may take the form of direct resistance to the growth of the pathogen in the stem base or of indirect tolerance to eyespot through resistance to lodging. The French variety Cappelle-Desprez has been a source of resistance for most UK wheat varieties and a new source of resistance in wild goat grass has been discovered recently. Use of fungicides for control of eyespot increased rapidly after the introduction of the highly effective MBC fungicides in the 1970s, but has decreased recently because UK populations of P. herpotrichoides are now predominantly resistant to MBC fungicides and alternative fungicides are more expensive. Historically, cultural methods, especially crop rotation, have been important in the control of eyespot. The importance of cultural control of eyespot is now increasing because more non-cereal break crops are being grown and fungicide use is declining.     

Epidemiology of eyespot (Pseudocercosporella herpotrichoides) on winter wheat, with particular reference to the period of infection

The period of infection by Pseudocercosporella herpotrichoides was investigated by transplanting winter wheat from uninfested to infested sites at Cambridge. Maximum infection was found on plants moved between November and March; most conidia were trapped during winter but infection was not closely related to the numbers of conidia. Plants exposed at intervals on infested land and incubated in a standard environment became infected throughout the period from October to July; infection was not closely related to the numbers of conidia trapped or plant age, but was positively correlated with the number of wet days/wk. Thus, while enough P. herpotrichoides conidia were dispersed throughout the life of winter wheat crops to cause substantial infection, environmental factors limited the development of new infections from April onwards. Practical implications of these results for the choice of sowing date, the development of resistant varieties, and fungicide use, are discussed.     

The effect of temperature on loose smut of wheat (Ustilago nuda)

In experiments made in controlled-temperature rooms on the wheat variety Kota, inoculated with two races of Ustilago nuda (Jens.) Rostr., maximum smut development occurred at 23 °C; at 20 ° and 15 °C the incidence and extent of smutting was reduced. An even greater reduction occurred when seedlings were grown at 6 °C for 2 months. No marked effect of temperature on mycelial development within the spike was observed until the ear emerged, when the mycelium in many of the ears of plants subjected to the lower temperatures failed to sporulate. The date of sowing in the field influenced disease development, presumably through the period of exposure to winter temperatures. Variety Little Club inoculated with race C₃ showed a similar but less marked response to cold-treatment.     

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.     

The effect of phenylacetic acid on ethylene formation in wheat seedlings

Phenylacetic acid (PAA) was found to induce ethylene formation in wheat coleoptile segments. In its most effective concentration (0.5 mM) PAA was by approximately 60 % less active than 0.1 mM indole-3-acetic acid (IAA). PAA-induced ethylene formation was stimulated with 0.1 mM L-methionine by 24 % and totally inhibited by 2.5 and 5 μ gml^-1 aminoethoxyvinylglycin (AVG) and 10 μg ml^-1 cycloheximide. Cyoloheximide in lower concentration (5 μg ml^-1) and actinomycin D (10 μg ml^-1) inhibited PAA-induced ethylene formation by 50 % and 40 %, respectively. After the simultaneous addition of PAA and IAA ethylene formation was by 35 % lower than in the presence of IAA itself. Further, the coleoptile segments preincubated in IAA and then incubated in PAA solution produced by 35 % less ethylene than those incubated in plain buffer after preincubation in IAA. Quite the opposite effect was found when the segments were preincubated in PAA and then transferred into IAA solution. This treatment resulted in 70 % stimulation of ethylene formation over segments preincubated in PAA and incubated in buffer.     

Predisposing effects of Eyespot (Pseudocercosporella herpotrichoides) on Septoria nodorum infection of winter wheat

All isolates of PeniciUium simplicissimum, P. verrucosum var. cyclopium, P. brevicompactum, P. multicolor, P. oxalicum, P. paxilli, Botrytis cinerea, and of a Gliocladium sp. obtained from necrotic virus-tested narcissus twin-scales previously dtoped in benomyl were tolerant to 1000 μg/ml of this fungicide in agar. Every necrotic twin-scale examined was infected with at least one of these species. The first two species were the most frequently isolated and a similar range of species infected different narcissus clones or different cultivars. It is argued that the use of benomyl during the twin-scaling programme should be discontinued as soon as a suitable alternative fungicide is found.     

The effect of water potential on soil-borne diseases of wheat seedlings

Under glasshouse conditions, Fusarium culmorum caused more injury to wheat seedlings in soil at 33% saturation (approximately – 1 bar) than at 66% (–0·1 bar). The same effect occurred with inoculated seeds raised on soil moisture tension tables at –0·1 and –0·5 bar, at 20° and at 15°C. In controls, spontaneous infection by soil-borne F. culmorum was unaffected by soil water potential. Inoculated F. nivale, at 127deg; and at 15°C, showed a similar, but smaller, effect to that with inoculated F. culmorum. Water potential in this range did not affect disease incidence with inoculated Gaeumannomyces graminis, but disease was more severe in the drier soil. On tension tables, seedlings did not consistently develop faster under wetter conditions, and factors other than host growth rate probably played a part in limiting fungal attack.     

Sensitivity to prochloraz in populations of the eyespot fungus, Pseudocercosporella herpotrichoides, in relation to fungicide treatments and their efficacy in continuous winter wheat

Eyespot was assessed and grain yields determined in the eighth and ninth years (1992 and 1993) of a field experiment in which the fungicides carbendazim and prochloraz were applied, separately or in mixtures, to plots of successive crops of winter wheat. Populations of the eyespot fungus, Pseudocercosporella herpotrichoides, were characterised by the proportions of cultures grown on agar from infected stems that were W-type or R-type, or were carbendazim-resistant or carbendazim-sensitive. Sensitivity to prochloraz in agar was determined for isolates from populations sampled in 1992 using restricted maximum likelihood analysis of EC50s (concentrations needed to decrease colony growth by 50%), to deal with the unbalanced data, and comparisons were made by Wald statistics. Control by prochloraz was maintained but in 1992, as in some previous years, its application with carbendazim was more effective than its application alone. Selection by prochloraz for strains sensitive to carbendazim continued to occur and may have contributed to its sometimes relatively poorer performance in the absence of carbendazim. R-type isolates taken from prochloraz-treated plots, with or without carbendazim, in 1992 were less sensitive to prochloraz than were isolates from plots untreated with prochloraz. There was evidence of a greater range of sensitivities to prochloraz in R-type than in W-type isolates (although some uncertainty results from the small numbers of W-type isolates obtained from some treatments), which would explain the selection of the R-type by prochloraz. The significance of these findings to modern wheat growing practices is considered.     

N(6)-Adenine DNA-methyltransferase in wheat seedlings

The N(6)-adenine DNA-methyltransferase was isolated from the vacuolar vesicle fraction of wheat coleoptiles. In the presence of S-adenosyl-L-methionine the enzyme de novo methylates the first adenine residue in the TGATCA sequence in the single- or double-stranded DNA substrates but it prefers single-stranded structures. Wheat adenine DNA-methyltransferase (wadmtase) is a Mg(2+)- or Ca(2+)-dependent enzyme with a maximum activity at pH 7.5-8.0. Wadmtase seems to be responsible for mitochondrial DNA modification that might be involved in the regulation of replication of mitochondria in plants.     

The effect of indole-3-Acetic acid on ethylene formation in wheat seedlings

Isoperoxidase B 1 isolated from winter wheat (Triticum aestivum L., cv. Jubilar) seedlings was shown to catalyze ethylene formation from α-keto, γ-methylmercaptobutyric acid (KMBA). In the presence of Mn²⁺, indole-3-acetic acid (IAA), andp-coumaric acid, the kinetics by isoperoxidase B 1 catalyzed conversion of KMBA into ethylene and other products was similar to that of IAA oxidation. The reaction rate was therefore controlled by IAA through its electrondonating properties. Exogenous IAA induced ethylene formation in the segments of etiolated wheat coleoptiles. IAA-induced ethylene production was enhanced by L-methionine and mitomycin C. Aminoethoxy-analogue of rhizobitoxine, ferulic acid, sodium benzoate, cycloheximide and actinomyoin D exhibited significant inhibitory effects. These data indicate that the overall reaction mechanism in coleoptile segments involves RNA and protein synthesis. The site of IAA action is not specific; 2,4-dichlorophenoxyacetic, α-naphthylacetic and indole-3-butyric acids, respectively, possessed comparable inductive effect as IAA. Indole-3-propionic acid, indole, L-tryptophan and glucobrassicin had only low inductive efficiency, and moreover indole and L-tryptophan slowed down IAA-induced ethylene formation.