Fifty-seven cultivars of zinnia (Zinnia elegans Jacq.) were studied for 17 weeks to determine their resistance to alternaria blight (Alternaria zinniae Pape), powdery mildew (Erysiphe cichoracearum DC ex Merat) and bacterial leaf & flower spot [Xanthomonas campestris pv. zinniae (syn. X. nigromaculans f. sp. zinniae Hopkins & Dowson)]. A disease severity scale was used to determine acceptability for landscape use. At week 4, all cultivars were acceptable. At week 10, eleven cultivars were acceptable. At week 17, all cultivars were unacceptable. Ten cultivars had been killed by one or more pathogens by week 17. Only two cultivars showed any tolerance to any disease (powdery mildew) at week 17.
Linda Gombert, Mark Windham, and Susan Hamilton
H.A.J. Hoitink and A.G. Stone
Many factors affect the potential for composts to provide biological control of diseases caused by soilborne plant pathogens. Heat exposure during composting kills or inactivates pathogens and weed seeds if the process is monitored properly. Unfortunately, most beneficial microorganisms also are killed by this heat treatment. Conditions must be provided after peak heating that enhance natural recolonization of composts by biocontrol agents. The raw feedstock, the environment in which the compost is produced, as well as conditions during curing and utilization, determine the potential for recolonization by this microflora and the induction of disease suppression. Controlled inoculation of compost with biocontrol agents has proved necessary to induce consistent levels of suppression on a commercial scale. Compost stability is another important factor. Immature composts serve as food for pathogens and increase disease even when biocontrol agents are present. On the other hand, excessively stabilized organic amendments, such as highly decomposed peats, do not support the activity of biocontrol agents and disease therefore develops. Finally, salinity, C to N ratio, and other factors affect suppressiveness. Each of these factors will be discussed.
Annelle W.B. Holder, Winston Elibox, and Pathmanathan Umaharan
., 1995 ). It is regarded as one of the major contributors to the demise of the anthurium industry in the Caribbean region ( Saddler et al., 1995 ), along with bacterial blight disease caused by X. axonopodis pv. dieffenbachiae ( Elibox and Umaharan
Nicolas Tremblay, Tarif Charbaji, Francois Fournier, and Odile Carisse
Scientific literature contains several examples of disease development influenced by fertilization practices. A set of data collected by the «Scouting and Research Network, South of Montreal Area» and consisting of disease and tissue analysis data on carrot and onion crops was made available for principal component analysis. It was hypothesized from the analysis that high N tissue levels would reduce Cercospora carotae and Botrytis squamosa importance on carrot and onion leaves, respectively. In a controlled environment study, Cercospora spots were inversely related to urea levels sprayed on carrot leaves although urea had no influence on plant growth. In a field study with onion, however, urea sprayed at 10 kg/ha, alone or in combination with a fungicide, had no effect either on Botrytis or on maturation or yield. With these mixed results, more research seems needed to assess the potential of nutrient sprays in reducing pesticide use.
J.H. Dunn, D.D. Minner, B.F. Fresenburg, and S.S. Bughrara
We evaluated the effect of fertilization treatments in combination with clippings disposal on perennial ryegrass (Lolium perenne L.) in two adjacent locations. Clippings left on turf during mowing decreased dollar spot (Sclerotinia homoeocarpa F.T. Bennett) in both locations during three summers compared with clippings removed in mower baskets. However, brown patch (Rhizoctonia solani Kuhn) increased during July and Aug. 1995 when clippings were left on turf. Dollar spot was more severe with N (kg·ha–1·year–1) at 120 compared to 240; brown patch was more severe at 240. While clippings disposal had significant effects on disease incidence, implementation may not be practical because of the contrary responses of the observed diseases to this management approach.
Arthur S. Greathead
The use of disease-free greenhouse-grown plug transplants for the establishment of field plantings of many vegetable crops in the arid west and southwestern regions of the United States has become a very important part of the agricultural system in these areas. The development of effective disease-control programs for use in the greenhouse involves a broad knowledge of production systems, water management, growing media, cultural techniques, etc., as well as knowledge of the discipline of plant pathology. The consultant in this field also must know the people and organizations with whom he is working. His goal is not simply the passing on of technical information, but also assisting in the incorporation of that information into the total growing program. Good communication skills and the development of an atmosphere of trust between all parties concerned are a vital part of the consultant's work.
E. Tanne, L. Kuznetsova, J. Cohen, S. Alexandrova, and A. Gera
Recently, yellows diseases have become more common in Israel, and phytoplasmas have been detected in some of these diseased crops. Commercial fields of two celosia species (Celosia plumosa L. and C. cristata L.) also have exhibited yellows symptoms and total crop failure. Typical mycoplasma-like bodies were observed in infected but not in healthy plants. The same plants were analyzed for the presence of phytoplasma by polymerase chain reaction (PCR), using the universal oligonucleotide pair r16SF2/r16SR2, followed by nested PCR using group-specific primers. Restriction analyses performed with these products indicated that two different types of phytoplasmas are infecting celosia. PCR-RFLP analysis of one type revealed a restriction pattern typical of aster yellows. Similar analysis of the second type indicated possible relatedness, though not identity, to the pattern of phytoplasmas of the Western-X group. This is, to our knowledge, the first report of phytoplasma infection in celosia.
Michael C. Long*, Stephen L. Krebs, and Stan C. Hokanson
Forty deciduous azalea (Rhododendron sp.) cultivars from commercial sources were evaluated for powdery mildew (Microsphaera sp.) resistance. Plants were established in two duplicate field plantings in Ohio and Minnesota and evaluated in 2002 and 2003. Plants were scored using a disease symptom rating based on the percent of leaf area infected, evaluating both ab- and adaxial leaf surfaces. Highly significant differences were found for cultivar, location, year, cultivar × location and cultivar × year for disease severity. Calendulaceum × speciosum, `Fragrant Star', `Garden Party', `Late Lady', `Millennium', `Parade', and `Popsicle' showed no powdery mildew symptoms in both locations. Another group of plants with only minimal symptoms (<25% leaf area affected) included `Jane Abbott', `Magic', `Northern Hi-Lights' and `Snowbird'. The symptom-free cultivars exhibited glaucous foliage, suggesting a potential, common resistance mechanism. The mean scores for the abaxial and adaxial leaf surfaces were 2.34 and 1.64, respectively, although four cultivars had more disease symptoms on the adaxial surface. `Arneson Gem' showed nearly a two-point difference between abaxial and adaxial scores. Evaluations of azalea powdery mildew susceptibility should consider both leaf surfaces and use the highest score as the best estimate of host resistance.
Mohamed F. Mohamed and Dermot P. Coyne
Common bacterial blight, incited by Xanthomonas campestris pv. phaseoli (Smith) Dye (Xcp), is a serious disease of common beans (Phaseolus vulgaris L.). Three experiments were conducted twice in growth chambers at 26 ± 1C under short (10 hours light/14 hours dark) and long (16 hours light/8 hours dark) photoperiods to determine the influence of these photoperiods, flower bud removal, pod development, and pre- and post-inoculation photoperiods on the reaction of common beans to Xcp. In one test, `PC-50' (susceptible; S) flowered earlier and was more susceptible to Xcp under the short photoperiod than under the long photoperiod. BAC-6 (resistant; R) flowered at the same time under both photoperiods but developed rapid leaf chlorosis (RLC) (hypersensitive reaction) under long photoperiods. Flowering and disease reactions to Xcp by XAN-159 (R) were similar under both photoperiods. In a second test, daily removal of flower buds of `PC-50' decreased its susceptibility to Xcp under the short photoperiod. RLC of inoculated leaves of BAC-6 occurred during flowering and pod development under both photoperiods. XAN-159 expressed a high level of resistance to Xcp but showed RLC at later pod development stages. In a third test, the disease reaction of `PC-50' was affected by the particular photoperiod applied post-inoculation but was not influenced by the photoperiod applied before inoculation with Xcp. The implications of these results in breeding beans for resistance to Xcp are discussed.
Blair Buckley III, James N. Moore, and John R. Clark
Rosette, incited by Cercosporella rubi (G. Wint.) Plakidas, is the most severe disease of blackberries in the southern United States. Sixteen blackberry cultivars and breeding selections were evaluated in a field test over a 3-year period for incidence and severity of rosette. Test plots were planted in a randomized complete block design with four replications. A plot consisted of a 3-m hedgerow of blackberry canes. Each test plot row was bordered on each side by a row of the rosette-susceptible cultivar Shawnee. Disease ratings were conducted on five random floricanes in each plot. Disease severity was rated with a 1 to 8 scale (1 = 0% floricane nodes with rosettes, 2 = 0% to 10%, 3 = 10% to 25%, 4 = 25% to 50%, 5 =50% to 75%, 6 = 75% to 90%, 7 = 90% to 100%, 8 = 100%). `Shawnee' and `Rosborough' had high incidence and severity. Cultivars and selections with moderate-high incidence and low-moderate severity were `Brazos', `Cheyenne', `Choctaw', A-1260, A-1442, A-1560, and A-1585. Cultivars and selections with zero-low incidence and severity were `Arapaho', `Humble', `Navaho', A-1374, A-1594, A-1616, and A-1617.