Interpretive Summaries

Long-Term Effect of Biological Soil Disinfestation on Verticillium Wilt. Jan-Kees C. Goud, Aad J. Termorshuizen, Wim J. Blok, and Ariena H. C. van Bruggen, Biological Farming Systems Group, Wageningen University, Marijkeweg 22, 6709 PG Wageningen, the Netherlands. Plant Dis. D-2004-0428-01R, 2004 (online). Accepted for publication 25 February 2004.

Because of the ban on methyl bromide, growers need alternative measures to control soilborne fungi and nematodes. Biological soil disinfestation (BSD), involving incorporation of grass combined with plastic mulching, eliminates many soilborne pests and diseases through the creation of anaerobic conditions. It is as effective as methyl bromide and has a comparable broad spectrum of activity. BSD was compared with a control treatment at two locations for the control of Verticillium wilt. After the soil treatments, plots were cropped with Acer platanoides and Catalpa bignonioides for 4 years. Soil inoculum levels of Verticillium dahliae were reduced by 85% after BSD and did not increase during the 4-year period. Less Verticillium wilt occurred, and stem infection with V. dahliae was reduced by 80 to 90%. Pratylenchus fallax populations in the soil and in roots were reduced by 95 to 99%. Market value of the plants was up to € 140,000 ha(^–1) higher for A. platanoides and up to € 190,000 ha(^–1) higher for C. bignonioides compared with the control. BSD is an effective, economically profitable, and environmentally friendly control method for tree culture and other high value crops.

Development of MILIONCAST, an Improved Model for Predicting Downy Mildew Sporulation on Onions. Tijs Gilles, Roy Kennedy, Kath Phelps, and John P. Clarkson, Horticulture Research International, Wellesbourne, Warwickshire CV35 9EF, UK. Plant Dis. D-2004-0419-01R, 2004 (online). Accepted for publication 30 January 2004.

Downy mildew, a common disease of onions, which is caused by the fungus Peronospora destructor, is spread by wind-dispersed spores. These spores are produced only during periods of high humidity at night. A new study was conducted to develop an improved sporulation model, because it was found that existing models frequently failed to predict sporulation. Laboratory studies found that spores are produced most quickly at 8 to 12°C after 5 h of high humidity at night. The largest numbers of spores are produced at 100% relative humidity (RH), and this decreases with decreasing humidity until only a few spores are produced at 93% RH. A model has been developed based on these laboratory findings, which has been named MILIONCAST, an acronym for MILdew on onION foreCAST. MILIONCAST was tested for its accuracy in predicting sporulation on onion plants under outdoor conditions, and 81% of predictions were correct. The existing models DOWNCAST, a version of DOWNCAST modified by de Visser, and ONIMIL gave accurate predictions in only 69, 74, and 67% of cases, respectively. The MILIONCAST sporulation model could become a useful tool in the management of onion downy mildew when it is used in combination with infection models.

Finger Imprint of Poncirus trifoliata: A Specific Interaction of a Viroid, a Host, and Irrigation. G. Vidalakis, D. J. Gumpf, J. A. Bash, and J. S. Semancik, Department of Plant Pathology, University of California, Riverside 92521. Plant Dis. D-2004-0426-01R, 2004 (online). Accepted for publication 26 February 2004.

The “finger imprint” symptom of citrus is unusual in the appearance of horizontal grooving patterns with the impression of a “strangulation” of the tree trunk. The infrequent observation of this symptom may be explained not only by the highly specific interaction between Citrus viroid IIIb and the trifoliate rootstock host, but also the requirement of water application by sprinkler irrigation directed against the base of the tree. Thus, the cultural practice of the manner of irrigation is as vital to the production of “finger imprint” as the viroid transmissible agent and the single host species known to express the symptom. Although dramatic in appearance, the symptom does not appear to cause any serious harm to the longevity of the tree or fruit yield and quality.

Soybean Cultivar and Foliar Fungicide Effects on Phomopsis sp. Seed Infection. J. A. Wrather, J. G. Shannon, and W. E. Stevens, University of Missouri-Delta Center, Portageville 63873; D. A. Sleper, University of Missouri-Columbia, Columbia 65211; and A. P. Arelli, United States Department of Agriculture–Agricultural Research Service, Jackson, TN 38301. Plant Dis. D-2004-0503-01R, 2004 (online). Accepted for publication 3 March 2004.

Phomopsis seed decay (PSD) caused by Phomopsis spp. can be severe when soybean seed producers in the southern United States use the early soybean production system (ESPS) to avoid late-July through early-September drought damage to soybean. The usefulness of this production system would be greater if developing seed could be protected from PSD by foliar application of fungicides or by planting Phomopsis spp.-resistant soybean lines. The objective of this research was to determine the effects of the fungicides Benlate and Quadris applied to soybean, at various times, on percent Phomopsis spp. infection of seed in Asgrow 3834, a PSD-susceptible cultivar, and SS93-6012, a PSD-resistant soybean line, planted in mid-April. The percent Phomopsis spp. infection of Asgrow 3834 seed averaged over years was significantly less for the Benlate (8 oz. of formulation/acre) applied at R3 + R5 treatment (48.6% seed infection) than the control (52.8% seed infection) and significantly greater for the Quadris (9.3 fl. oz. of formulation/acre) applied at R3 + R5 treatment (61.6% seed infection) than the control (52.8% seed infection). This method of managing PSD will not be acceptable to soybean growers. The percentage of Phomopsis spp. infection of Asgrow 3834 seed averaged over years (52.8% seed infection) was significantly greater than for line SS93-6012 (2.8% seed infection). There were no differences in percent Phomopsis spp. infection of SS93-6012 seed between the control (2.8% seed infection) and Benlate treatment (4.0% seed infection). The most effective method for PSD management was to plant a resistant soybean line. Line SS93-6012 will be useful in breeding programs focused on developing high-yielding PSD-resistant cultivars.

Evaluation of Systems for Timing of Fungicide Sprays for Control of Postbloom Fruit Drop of Citrus in Brazil. N. A. R. Peres, Former Ph.D. student, N. L. Souza, Professor, and E. L. Furtado, Professor, Universidade Estadual Paulista, FCA, Botucatu, SP, 18603-970, Brazil; and L. W. Timmer, Professor, University of Florida, Citrus Research and Education Center, Lake Alfred 33850. Plant Dis. D-2004-0511-02R, 2004 (online). Accepted for publication 15 March 2004.

Postbloom fruit drop (PFD) of citrus, caused by Colletotrichum acutatum, infects petals of citrus flowers and induces fruit drop and the retention of flower bases. The disease can cause severe yield losses depending on weather conditions during the bloom period. Good disease control usually is achieved only with fungicide applications at proper timing. In Florida, a predictive model was developed for timing of fungicide applications considering the amount of disease present, rainfall for the last 5 days, and leaf wetness duration. A new system, the PFD-fungicide application decision system (PFD-FAD), was developed including other risk factors, such as the previous history of PFD in the grove, the susceptibility of the variety, the stage of the bloom, and the same factors used in the Florida model. In this study, we compared the Florida PFD model (from 1999 to 2002) and the PFD-FAD (in 2001 and 2002) with a grower’s choice program and a currently recommended flower development-based system in groves in southern São Paulo state in Brazil. The use of the Florida PFD model saved two sprays compared with the flower development-based program in 1999 and saved one spray compared with the flower development-based program and the grower’s choice in 2000. PFD was severe in 2001 and the Florida PFD model, the flower development-based program, and the grower’s choice reduced disease and improved fruit yields with two to three applications, but the PFD-FAD achieved similar yields with only one spray. No yield loss occurred in 2002 and the Florida PFD model and the PFD-FAD saved one spray compared with the other systems. The results showed that the Florida PFD model and the PFD-FAD were effective for timing fungicide applications to control PFD in Brazil. The use of the PFD-FAD is simpler and the system can be a valuable tool for timing of fungicide applications for a better disease management. The PFD-FAD system can be accessed at http://infotech.ifas.ufl.edu/disc/pfd.

Overwintering of Conidia of Venturia inaequalis and the Contribution to Early Epidemics of Apple Scab. I. J. Holb, Department of Plant Protection, Centre for Agricultural Sciences, University of Debrecen, P.O. Box 36, H-4015 Debrecen, Hungary; B. Heijne, Wageningen University and Research Centre, Applied Plant Research, P.O. Box 200, 6670 AE Zetten, The Netherlands; and M. J. Jeger, Department of Agricultural Sciences, Imperial College of Science, Technology and Medicine, Wye, Ashford, Kent TN25 5AH, UK. Plant Dis. D-2004-0518-01R, 2004 (online). Accepted for publication 15 March 2004.

Overwintering of conidia of Venturia inaequalis associated with shoots and buds was determined, and its contribution to early spring epidemics of apple scab was evaluated during three consecutive seasons (1999 to 2001) in the Netherlands. The proportion of shoots with superficial black mycelia or conidia was greater than 65%, and the mean number of conidia on a 1-cm piece of shoot length ranged from 581 to 1,033. The viability of conidia on shoots was less than 1.5%. No macroscopic scab lesions were detected on the scales of dormant buds. However, microscopic examinations of individual bud tissues demonstrated that the number of conidia was greater than 3,000 per 100 buds in each year. The mean viability of conidia associated with buds was less than 2% and ranged from 4 to 11% for the outer and inner bud tissues, respectively. Results of field assessments at tight cluster phenological stage showed that the percentage of infection caused by the viable overwintered conidia was less than 4%. Our results indicated that conidia were unlikely to overwinter on the surface of apple tissues (such as shoots or outer bud tissues) where exposed to fluctuating environmental conditions; and, consequently, conidia were unlikely to play a role in initiating an early epidemic of apple scab in the spring. However, our results indicated a risk from overwintered conidia in the inner bud tissues arising from a high level of scab the previous autumn. Therefore, orchards with high levels of apple scab, where ascosporic inoculum is much reduced, e.g., by sanitation, should be protected in early spring by means of fungicide treatment at green tip.