Stem Lesion Length Research Articles

Genetic variation for resistance to Sclerotinia head rot in sunflower inbred lines

Sclerotinia sclerotiorum is one of the most devastating pathogens of sunflower ( Helianthus annuus), causing substantial yield losses. This study was conducted to screen sunflower lines for their reaction to S. sclerotiorum using artificial inoculation of sunflower heads. A set of 15 maintainer and 30 restorer lines was screened in four years by inoculating the heads with mycelium grown on millet seed. Susceptibility was assessed by measuring the lesion length of the head and scoring the part of the head that was rotted. With the exception in 1999, with no results owing to an infection rate less than 50%, the test proved to be effective and practicable. Significant genotypic variances among lines and moderate heritabilities were observed for the resistance traits. No differences between the means of maintainer and restorer lines were found. While lesion length of the head showed no differences between branched and unbranched restorer lines, significantly higher means for head rot were observed for branched lines. Both scoring traits were moderately correlated and showed a weak correlation to stem lesion length after leaf inoculation. Inbreds with high resistance were identified that can be used as parents in resistance breeding programs.

First Report of Phomopsis longicolla from Velvetleaf Causing Stem Lesions on Inoculated Soybean and Velvetleaf Plants.

Reddish brown lesions were observed on the lower stem and upper root area of velvetleaf (Abutilon theophrasti Medik.) plants growing in an Illinois soybean field in June 2000. The lesions were similar in appearance to those caused by Rhizoctonia root rot of soybean. Stems and roots with lesions were cut into ≍5-mm pieces, surface-disinfested, and placed on 2% water agar at pH 4.5. The cultural morphology of the two isolates fit the description of Phomopsis longicolla Hobbs (1). Colonies on potato dextrose agar (PDA) were floccose, dense, and white. The undersides of the cultures were colorless. Stromata were large, black, and spreading. The pattern of stromata in one isolate was effuse, and most of the stromata were immersed or semiimmersed in the medium, whereas the stromata from the other isolate were massive and prominent. Neither isolate turned green on PDA. Alpha conidia were hyaline, ellipsoidal to fusiform, and guttulate. DAPI (4',6-diamidino-2-phenylindole)-stained alpha conidia were uninucleate. Beta conidia and perithecia did not occur on either PDA or oat flakes on water agar from 1 to 10 weeks at 25°C under a 12-h photoperiod. The DNA sequences of the mitochondrial small subunit rRNA genes of the two isolates were identical and shared 100% sequence identity with two P. longicolla soybean isolates that we had identified previously. Pathogenicity tests were conducted in a greenhouse by cutting the stems of 3-week-old soybean and velvetleaf plants at the second internode. Mycelial plugs (4 mm in diameter) from the margin of 1-week-old cultures of the two isolates from velvetleaf and one from soybean were individually placed mycelial side down directly on the top of cut stems of 10 to 15 plants per isolate. Controls included noninoculated plants with and without PDA plugs. Plants were kept in a mist chamber in the dark at 25°C for 4 days and were then transferred to a greenhouse with a 16-h photoperiod at 24 ± 3°C. Stem lesions were measured 7 days after inoculation. The experiment was repeated once. Mean stem lesion lengths caused by the velvetleaf and soybean isolates were 23 and 20 mm, respectively, on soybean stems, while negative controls produced no lesions. Mean stem lesion lengths caused by the velvetleaf and soybean isolates were 23.5 and 12 mm, respectively, on velvetleaf stems. P. longicolla was reisolated from the stem lesions of five randomly collected plants. This is the first report of P. longicolla being isolated from velvetleaf and causing stem lesions on inoculated soybean and velvetleaf plants. Reference: (1) T. W. Hobbs et al. Mycologia 77: 535, 1985.

Chemical control of Phytophthora cinnamomi in potted Leucospermum plants

The efficacy of fosetyl-Al (soil drench), metalaxyl-M (soil drench), fluazinam (soil drench) and potassium phosphonate (foliar spray) in controlling Phytophthora cinnamomi root infections of Leucospermum cvv. High Gold, Sunrise and Spider was compared. The Leucospermum plants were potted, placed in a fibreglass tunnel and inoculated. Fungicide treatments began 4 weeks after inoculation. Stem lesion length was measured and the degree of leaf chlorosis as well as root rot were rated at the conclusion of the experiment. The pathogen was re-isolated from the root and crown of each plant, and soil baiting and dilutions were done to monitor soil populations of P cinnamomi. During the 1997 season, metalaxyl-M gave the best control compared with the other treatments. The combination of metalaxyl-M with cv. Spider completely suppressed symptom development. However, during the 1998 season when climatic conditions were favourable for disease development, no differences were observed between the treatments and the control. Cv. Spider had higher tolerance against the fungus even with the most aggressive isolate and during the warmer 1998 season, indicating the importance of combining chemical control with plant resistance. None of the treatments showed any phytotoxicity.

Resistance to Phytophthora citrophthora and P. parasitica and Nursery Characteristics of Several Citrus Rootstocks.

Studies were conducted to compare existing and potential citrus rootstocks with respect to resistance to root rot and gummosis caused by Phytophthora citrophthora and P. parasitica in greenhouse and growth chamber experiments and horticultural performance under simulated nursery conditions. Depending upon rootstock and experiment, mean root weights resulting from inoculation with P. citrophthora were 27 to 96% lower than the comparable controls. In similar experiments with the same rootstocks, inoculation with P. parasitica resulted in root weights that were 38 to 95% less than weights of the noninoculated controls. During 1994 or 1995, mean root weight reduction compared with noninoculated plants among Citrus macrophylla, rough lemon, C. volkameriana, and Sunki mandarin × Flying Dragon trifoliate (62-109-19) attributable to P. citrophthora and mean root weight reduction among C. macrophylla, C. volkameriana, rough lemon, Sacaton citrumelo, Sunki mandarin × Flying Dragon trifoliate (62-109-19), African shaddock × Rubidoux trifoliate, and Shekwasha mandarin × English trifoliate attributable to P. parasitica were significantly less than those recorded for all other tested rootstocks. Rootstocks that sustained a low percentage of root weight reduction generally experienced a low percentage of shoot weight reduction and survived longer as well. In evaluation of resistance to gummosis, depending on rootstock and experiment, the mean length of stem lesions caused by P. citrophthora on rootstocks ranged from 0.2 to 25.0 mm, whereas values for P. parasitica ranged from 0.2 to 18.5 mm. Stem lesions smaller than 5 mm in length were recorded for 21 and 14 of 36 different rootstocks inoculated with P. citrophthora and P. parasitica, respectively. On the other hand, P. citrophthora and P. parasitica caused stem lesions of at least 10 mm in length on 8 and 16 citrus rootstocks, respectively. Desirable nursery characteristics, including vigorous growth, minimal branching, and high leaf chlorophyll content, were demonstrated most prominently by Gomiri rough lemon, C. volkameriana, and Benton citrange, and to a lesser degree by some other rootstocks. Possible factors that could account for inconsistent classification of some citrus rootstocks as susceptible or resistant to Phytophthora root rot and gummosis are discussed.

Control of Phytophthora root and crown rot of bell pepper with composts and soil amendments in the greenhouse

Ten composts and soil amendments were evaluated in the greenhouse for control of Phytophthora root and crown rot of bell pepper caused by Phytophthora capsici. Three rates of calcium humate, calcium sulfate, chitosan, citrus pulp, citrus pulp with molasses, crab shell waste, humate, mustard residue, composted sewage sludge-yard trimming, and wood chips were incorporated into soil in pots prior to transplanting 6-week-old bell pepper cultivar Early Calwonder. P. capsici zoospores (2 × 10 4) were placed into the soil of each pot 2 weeks after transplanting. Areas under the disease progress curves (AUDPC) for disease severity and stem lesion length were determined, and final root and shoot dry weights were also evaluated. Soil populations of five microbial functional groups, and total microbial activity, measured by fluorescein diacetate hydrolysis analysis, were evaluated. Certain rates of chitosan, crab shell waste, and citrus pulp with molasses significantly reduced disease severity and/or stem lesion length, and increased root or shoot dry weights, compared with unamended controls. None of the other treatments were effective in reducing root and crown rot of bell pepper. Citrus pulp with molasses was unique in enhancing the total microbial activity and populations of certain microbial functional groups, compared with unamended controls.

A male-fertile Lycopersicon esculentum × Solanum lycopersicoides hybrid enables direct backcrossing to tomato at the diploid level

Solanum lycopersicoides Dun. is a wild nightshade native to Chile and Peru that possesses many traits of potential interest to tomato breeders, including environmental stress tolerance, resistances to disease and insect pests, and certain fruit quality characteristics. Sexual and somatic hybrids with the cultivated tomato (Lycopersicon esculentum Mill.) are readily obtained, yet have a strong tendency towards male-sterility combined with unilateral incompatibility, barriers that have deterred breeding efforts in the past. We report herein the synthesis of a partially male-fertile F1 hybrid by sexual crosses between tomato and a recently collected population of S. lycopersicoides. Over 280 BC1 plants were obtained by directed backcrossing to tomato at the diploid level, of which 58 were sufficiently fertile to permit selfing or additional backcrosses. The transmission of S. lycopersicoides genetic markers into a uniform L. esculentum background was confirmed with 24 isozyme or morphological loci on 11 of the 12 alien chromosomes. Potentially useful tolerance to gray mold (Botrytis cinerea) was demonstrated by inoculation of stem cuttings with mycelial plugs: at 6 days post-inoculation, the intergeneric hybrid showed little evidence of disease progression and the length of stem lesions averaged only one third that of susceptible controls.

Effects of temperature and pathogen isolate on laboratory screening of soybean for resistance to Sclerotina sclerotiorum

The effects of temperature and pathogen isolate on laboratory screening of soybean (Glycine max. L.) for resistance to Sclerotinia sclerotiorum were investigated. The screening procedure used excised stems from 5-wk-old plants. Temperature and isolate significantly affected the length of stem lesions on 15 and 10 cultivars, respectively. At 15 °C, lesions were significantly shorter, but there was no difference in lesion length between 20 and 25 °C. One of the isolates caused significantly shorter lesions. There were significant differences among cultivars, but there were no temperature × cultivar or isolate × cultivar interactions. Choice of isolate and temperature between 15 and 25 °C, therefore, should not affect the relative differences among cultivars when screened for resistance using the excised stem method. Maple Presto, Maple Arrow and Maple Ridge were the most resistant of 19 cultivars evaluated. Key words: Soybean, resistance, temperature, isolate, Sclerotinia sclerotiorum

Comparison of Laboratory and Field Evaluations of Resistance in Soybean toSclerotinia sclerotiorum

Fifteen commercial soybean (Glycine max) cultivars were evaluated for resistance to Sclerotinia sclerotiorum in the laboratory, using an excised stem technique, and in the feld during 1988 and 1989. In the laboratory tests, significant differences in the length of stem lesions among cultivars were observed. Maple Presto, McCall, and Clay were significantly more resistant to stem decay then Pride B152, Apache, Merit, and Portage. Maple Presto, and McCall were the most resistant (...)