Assessment of yield loss caused by common root rot in wheat cultivars in Queensland

The effects of common root rot (CRR) caused by Bipolaris sorokiniana on grain yield, number of tillers, number of grains and grain weight of wheat plants were determined in four field experiments. Sites with different soil populations of B. sorokiniana were selected and inoculum of the fungus added to some plots. Disease and yield measurements were made on eight cultivars and lines differing in susceptibility to CRR. Timgalen, Songlen and Hartog were susceptible whereas Kite, 1008 C16, 141-4 and ISWYN 32 were partially resistant to CRR. Grain yield, tiller and grain number, but not grain weight decreased as disease severity increased. Diseased plants had lower tiller numbers than healthy ones and as a consequence a reduced number of grains and grain yield per plant. Five methods were compared for estimating yield loss caused by the disease. Polynomial regression equations for each cultivar between yield and disease rating of sub-crown internodes or multiple regression equations between yield and disease parameters of sub-crown internodes or tiller bases were established. A third method involved the projection of yield losses from one cultivar to other cultivars and in a fourth method yield losses were estimated from actual yields. In addition, an equation Yield loss (%) = 3 46 + 0.23 disease severity) (%) was established in one experiment and used as a fifth method in the other experiments. Yield losses estimated by methods 1 and 2 were similar and higher than those from the other methods. In areas where disease severity is high, methods 1 and 5 appear to be the most suitable for determining yield losses. Losses in a susceptible cultivar, Timgalen, varied between 13.9 and 23.9% whereas those in a partially resistant cultivar, 1008 C16, varied between 6.8 and 13.6%.

Inoculum density of Cochliobolus sativus in soil and common root rot of wheat cultivars in Queensland

Common root rot was assessed in wheat cultivars grown at field sites in Queensland having low, intermediate and high inoculum densities of Cochliobolus sativus. The densities at seeding time in the upper 10 cm of the profile, were 3, 77 and 132 viable propagules per g soil respectively. Both incidence and intensity of disease at tillering, flowering and firm dough stages of plant development appeared closely related to inoculum density. Additional inoculum applied with seed increased disease markedly at the low inoculum density site and slightly at the intermediate site, and had no effect at the high inoculum density site, indicating a threshold level for maximum disease in the test. Reduction in dry weight of plants at the low and high sites paralleled the disease levels at the tillering and flowering stages. The relative reactions of the cultivars were expressed consistently: Songlen and Timgalen invariably were more diseased than Kite and QT2338/9. The results suggest that cultivars may be evaluated for resistance in the field from tillering onward if disease levels are adequate. C. sativus was the dominant fungus isolated from the subcrown internodes of plants from non-inoculated subplots. Its frequency of isolation was much less from the low inoculum density site than the intermediate and high inoculum density sites, and lower from Kite and QT2338/9 than from Songlen and Timgalen. Fursariurn chlamydosporum was isolated from subcrown internodes with increasing frequency as the season progressed. However, it was considered to be a secondary invader of the plant material.

Multiple infections of subcrown internodes of wheat (Triticum aestivum) by common root rot fungi

In the greenhouse, subcrown internodes of wheat plants were inoculated with Cochliobolus sativus (Ito & Kurib.) Drechsler ex Dastur, Fusarium culmorum (W. G. Smith) Sacc., F. acuminatum Ell. & Ev., or F. sulplureum Schlecht. and at various times subsequently with C. sativus; or the internodes were inoculated with C. sativus and subsequently with it or a Fusarium sp. Using isolation of the fungi as the criterion for infection, it was found that prepossession of the internode by C. sativus did not prevent later invasion by it or by the fusaria; however, C. sativus was a relatively unsuccessful challenger to F. culmorum and F. acuminatum. The results suggest that when C. sativus and one of these two Fusarium spp. appear together in isolations from common root rot diseased plants that the initial invader was C. sativus.

The effect of common root rot on components of grain yield in Manitou wheat

The effects of common root rot (Cochliobolus sativus) on components of grain yield in naturally infected Triticum aestivum cultivar Manitou were studied at Matador, Saskatchewan, by sampling plants at maturity in 1969, 1970, and 1971. Plants were sorted into severe (SE), moderate (MO), slight (SL), and clean (CL) categories based mainly on the extent of lesions on the subcrown internodes. The number of tillers per plant, the number and weight of grains per head, the weight per head, and the 1000-kernel weight in each category were determined. Increasing values of all five components were consistently associated with decreasing disease severity. SE was mostly significantly different from the other three categories in all components except 1000-kernel weight; differences between SL and MO were usually non-significant. CL and SL were mostly significantly different for the number of tillers per plant and weight per head but non-significant for the weight and number of grains per head and 1000-kernel weight. Apparently, the major effect of common root rot was to reduce the number of tillers per plant and number of grains per head.

Histology of Cochliobolus sativus infection in subcrown internodes of wheat and barley

Histological study of the infection process of Cochliobolus sativus in subcrown internodes of postseedling plants failed to disclose differences in wheat and barley cultivars that differed markedly in resistance to common root rot in the field. Appressoria and infection cushions were formed on the host surface before penetration. The dome-shaped infection cushions consisted of short-celled hyphae surrounded by long-celled running hyphae. Penetration occurred by fine infection pegs produced from appressoria or the short-celled hyphae in the infection cushions. A lignituber occurred beneath each penetration site. The fungus penetrated wheat or barley subcrown internodes via various epidermal cells, including hair cells, stomatal guard cells, and the rectangular cells. Infection proceeded from the epidermis to the cortex and endodermis, resulting in breakdown of these tissues. Sometimes the stele also was invaded and vascular tissues were occluded. Three morphologically different hyphae, namely fine aseptate infection hyphae, long-celled running hyphae, and coarse, short-celled hyphae, were observed. Dark-stained objects, directly or indirectly associated with the fungus, frequently were found in infected tissues.

The epidemiology of common root rot in Manitou wheat. III. Development of lesions on subcrown internodes and the effect of added phosphate

The development of common root rot lesions on subcrown internodes of Manitou wheat was followed in growth-chamber tests. Visible lesions caused by Cochliobolus sativus appeared 3 to 5 days after inoculation. The vertical spread of subcrown internode lesions was much faster than the lateral spread; normally, lateral spread did not start until maximum vertical extension had occurred. Considerable variation in rate of development of lesions was evident between individual plants. The overall mean daily rate of vertical extension of subcrown internode lesions was in the range of 0.3 to 0.5 mm. The rate was faster immediately after infection and declined with time. Lesion development on plants generally was more rapid in low-phosphate soil than in high-phosphate soil. This finding was exemplified by higher percentages of severely diseased plants at the end of the experiments; higher probabilities of transfer of plants from less to more severe categories in the intervals between observations; higher disease ratings; and higher mean daily rates of linear extension of lesions in low- than high-phosphate soil. The possible application of some techniques in developing a model for forecasting severity of disease is discussed.

The epidemiology of common root rot in Manitou wheat. II. Effects of treatments, particularly phosphate fertilizer, on incidence and intensity of disease

The effects of various treatments such as herbicide, irrigation, nitrogen, phosphate, and combinations of nitrogen and phosphate on common root rot (Cochliobolus sativus) in naturally infected Triticum aestivum cultivar ‘Manitou’ were studied at Matador, Saskatchewan, by sampling plants at intervals during 1969, 1970, and 1971. The treatments were compared for each of the following variables : number of diseased plants per square metre, percentages of diseased plants, and percentage disease ratings. Usually, disease assessed by each variable was higher in the checks than in the phosphate treatments. The effect of phosphate appeared maximal at midseason (growth stage 9–10.5). The differences between treatments such as herbicide, nitrogen, and irrigation and the checks were insignificant. A comparison with the results of other workers suggests interaction between phosphate and soil type, and between phosphate and cultivar.

Ultrastructure of somatic mitosis in a diploid strain of the plant pathogenic fungus Cochliobolus sativus

An ultrastructural study of mitosis in a diploid strain of Cochliobolus sativus showed the event to be intranuclear. Two nucleoli occasionally were present in interphase nuclei. During division the spindle pole body peripheral to the nuclear envelope divided; spindle microtubules radiated into the nucleoplasm from the amorphous granular region abutting the nuclear envelope beneath the bodies; chromosomes condensed at prophase, approached the equatorial plane at metaphase, and moved asynchronously at anaphase; single microtubules appeared attached to kinetochore-like structures. At telophase, nuclei exhibited maximal elongation; fissures of the nuclear envelope appeared in the interzonal region; the nucleolus dispersed. The polar nuclear areas became new daughter nuclei with nucleoli.

Somatic mitosis in haploid and diploid strains of Cochliobolus sativus

From examination of living and doubly stained nuclei, somatic nuclear division in a haploid and a diploid strain of Cochliobolus sativus resembles conventional mitosis in that a metaphase-plate-like configuration and an associated fusiform spindle were present. However, a spindle pole body located at the spindle poles, the nucleolus, and presumably, the nuclear envelope persisted throughout the division cycle. During division, chromosomes contracted and the spindle elongated. At anaphase, lagging chromosomes were invariably observed.Within multinucleate cells there was variation in the degree of synchrony of nuclear division. Most, however, showed partial synchrony with the gradient direction basipetal in the diploid, acropetal in the haploid. Diploid and haploid nuclei also differed in size, in the size of spindle, number and probably in the size of chromatinic elements, and sometimes in the number of nucleoli. The haploid number of chromosomes appears to be six to eight.

The epidemiology of common root rot in Manitou wheat: disease progression during the growing season

Common root rot in Triticum aestivum cultivar Manitou caused primarily by Cochliobolus sativus was followed during plant development in 1969, 1970, and 1971 at Matador, Saskatchewan. Plants were sampled at intervals, and three variables based mainly on the occurrence of lesions on subcrown internodes were studied: number of diseased plants per square meter; percentage of diseased plants; and disease rating which integrated percentage of diseased plants and disease severity on each plant. All variables increased with time, and the progression curves in all 3 years were hyperbolic, indicating that the increases were like those of a simple interest disease as described by Van der Plank. In two of the years, almost 100% of the plants were diseased considerably before the end of the season. The transformation proposed by Van der Plank for simple interest diseases, log10[1/(1 − x)], was applied to the percentages of diseased plants, and regressions were calculated. The slopes of these lines (infection rates) were as follows: 1969, 0.99% plants per day; 1970, 1.32%; and 1971, 1.96%. In 1969 the onset of disease was later than in 1970 and 1971, and there was correspondingly less disease at the end of the growing season.

COCHLIOBOLUS SATIVUS: V. HETEROKARYOSIS AND PARASEXUALITY

Multiply marked strains of Cochliobolus sativus were used to synthesize heterokaryons. The heterokaryons apparently were formed by hyphal anastomosis and nuclear migration. Some conidia (not more than 6%) from heterokaryotic cultures perpetuated the heterokaryons. Probably this low frequency of heterokaryotic conidia was due to a non-random distribution of nuclei in the conidia. Variation in strains due to heterokaryosis may be reduced by the isolation of single spores or hyphal tips in several successive generations. Heterozygous diploids were selected from two heterokaryons on the basis of their phenotypic differences: heterokaryons grew slowly and intermittently; diploids grew rapidly and uniformly. Heterokaryons but not diploids readily dissociated into their components. Diploid spores were consistently larger but not twice as large in volume as haploid ones.Dominant–recessive relationships were expressed in heterokaryons and diploids; white was recessive to tan spore color; nutritional requirements were recessive to the wild type alleles; and resistance to anisomycin was semidominant. Segregants were obtained from a diploid strain by various techniques. Equal numbers of heterozygous and "presumptive" homozygous segregants were found and most of them were recombinants. A parasexual cycle therefore occurs in this heterothallic ascomycete.

COCHLIOBOLUS SATIVUS: IV. DRUG-RESISTANT, COLOR, AND NUTRITIONALLY EXACTING MUTANTS

Auxotrophic mutants of Cochliobolus sativus were obtained from survivors of ultraviolet radiation by a modified total-isolation technique. Five or six hyphal tip isolations made from each survivor were tested for nutritional deficiencies. Although 0.48% of the survivors yielded auxotrophs, only about one-third of the hyphal tip isolations from these survivors were auxotrophic. Apparently, mutation in a multinucleate propagule resulted in a heterokaryotic culture and only some of the isolations from a culture were homokaryotic for the mutation. Some of the mutants were morphologically distinct from their parent and one, a methionineless strain, had white spores. Results indicated that recurrent requirements for growth occurred at different mutational sites.A strain resistant to the antibiotic anisomycin appeared as a spontaneous mutation. This strain grew at 1500 and its spores germinated at 1750 p.p.m.; wild-type isolates grew only at 75 and their spores germinated at 100 p.p.m. of the drug. The resistant mutant was pathogenic to wheat seedlings.

COCHLIOBOLUS SATIVUS: III. EFFECT OF ULTRAVIOLET RADIATION

The lethal and mutagenic effect of ultraviolet light on two isolates of Cochliobolus sativus was studied. Conidia of a dark-spored strain were considerably more resistant to high doses of radiation than those of a white-spored strain. Ultraviolet radiation increased the frequency of mutation in the fungus. Mutants were recovered that differed from their parents in degree of sporulation, rate of growth, pigmentation, amount of aerial mycelium, topography and consistency of colony, and pathogenicity to wheat seedlings. Tetrads from crosses between weakly and highly pathogenic strains indicated that pathogenicity was not a clearly segregating character.

STUDIES ON THE PERFECT STAGE OF HELMINTHOSPORIUM SATIVUM

Cochlioboly sativus, the perfect stage of Helminthosporium sativum, developed in some paired cultures of monoconidial isolates. A satisfactory medium for the propagation of perithecia consists of kernels of barley placed on Sach's nutrient agar. The barley was prepared by surface sterilization, followed by boiling to kill the embryos. Cultural studies of monoascosporic isolates showed that the fungus is heterothallic in the sense that the isolates are hermaphroditic, self-sterile, intragroup sterile, and intergroup fertile. The change from a conidial to a mycelial-type colony in one isolate was shown to be a true mutation. The two compatibility groups in C. sativus were shown to be randomly distributed in nature. Isolates of the fungus from any one area were observed to fall into either compatibility group. A comparison of the author's C. sativus with the description of Ophiobolus sativus given by Ito and Kuribayashi led to the conclusion that these fungi are identical.