Analysis of summer epidemic progress of apple scab at different apple production systems in the Netherlands and hungary.

ABSTRACT Two, 4-year studies on summer epidemic progress of apple scab were conducted at Randwijk, the Netherlands, from 1998 until 2001 and at Eperjeske, Hungary, from 2000 until 2003. Disease assessments were made on scab-susceptible cv. Jonagold. A range of nonlinear growth functions were fitted to a total of 96 disease progress curves (3 treatment classes x 2 plant parts x 2 disease measures x 4 years x 2 locations) of apple scab incidence and severity. The three-parameter logistic model gave the most consistent fit across three treatment classes in the experiment (integrated, organic-sprayed, and organic-unsprayed). Parameters estimated or calculated from the three-parameter logistic function were used to analyze disease progress. These were disease incidence and severity on the day of the first assessment (Y(s)); final disease incidence or upper asymptote for incidence (Y(if)) or severity (Y(sf)); fruit incidence and severity on day 40, after which no new lesions on fruits appeared (Y(40)); leaf incidence and severity on day 75, at which shoot growth stopped (Y (75)); relative (beta) and "absolute" (theta) rates of disease progress; inflection point (M); and area under the disease progress curve (AUDPC(S)) standardized by the duration of the total epidemic. Comparisons among disease progress curves were made by correlation and factor analysis followed by Varimax rotation. There were large differences but high positive correlations among the parameters Y(s), Y(f), theta, and AUDPC(S) across the three treatment classes. In the factor analysis, two factors accounted for more than 85% of the total variance for both incidence and severity. Factor 1 gave an overall description of epidemic progress of both scab incidence and severity and included the parameters Y(f), Y(40), Y(75), theta, and AUDPC(S). Factor 2 identified a relationship between the relative rate parameter (beta) and the inflection point (M) for severity and a relationship between disease incidence and severity. For an integrated or an organic orchard, theta, AUDPC(S), and one of Y(f) or Y(75) (because of the link with host phenology) can characterize apple scab epidemics during summer. Based on these findings, improved scab management approaches were provided for integrated and organic apple production systems.

Effect of natural windbreaks on drift reduction in orchard spraying.

In the Netherlands windbreaks are commonly grown to protect orchards against wind damage and to improve micro-climate. Natural windbreaks of broad-leaved trees can also reduce the risk of surface water contamination caused by spray drift during orchard spraying. Spray drift from pesticide applications is a major concern in the Netherlands, especially drift into water courses. So far, several drift reducing measures have been accepted by water quality control organisations and the Board for the Authorization of Pesticides (CTB), e.g. presence of a windbreak (i.e. 70% drift reduction at early season and 90% drift reduction at full leaf, respectively before and after first of May). From the experiments it was concluded that the risk of drift contamination is high during the early developmental stages of the growing season. The 70% drift reduction at early season as determined in previous experiments, appears to be valid only for windbreaks with a certain degree of developed leaves. At full leaf stage 80-90% drift reduction by the windbreak was measured. The use of evergreen windbreaks or wind-break species that develop in early season can reduce the risk of drift contamination considerably. Also, the combination of drift reducing methods, such as one-sided spraying of the last tree row and a windbreak is an effective method to reduce spray drift in the Netherlands in early season.

Dead flower buds of pear: effect of tree growth control, and Alternaria alternata as causal agent.

Dead (dormant) flower buds of pear is an important phenomenon in pear production in the Netherlands. Vigourous or unbalanced tree growth and Pseudomonas syringae pv. syringae are mentioned as likely causes of dead flower buds. Several tree growth control treatments including ethephon. Regalis (Prohexadione-Ca) and root pruning were evaluated. Regalis increased disease incidence. The plant stimulant (foliar fertilizer) Resistim (potassium phosphonate) reduced disease incidence. Pseudomonas syringae pv. syringae was occasionally isolated from diseased flower buds. However, Alternaria alternata was nearly always isolated from diseased buds. Indicating a strong relation between this fungus and dead flower buds of pear.

Overwintering of Conidia of Venturia inaequalis and the Contribution to Early Epidemics of Apple Scab.

Overwintering of conidia of Venturia inaequalis associated with shoots and buds was determined, and the contribution to early spring epidemics of apple scab was evaluated during three consecutive seasons (1999 to 2001) in the Netherlands. Examinations of shoot samples collected before bud break showed that the percentage of shoots with superficial black fungal mycelia or conidia was above 65%, and the mean number of conidia on a 1-cm piece of shoot length ranged from 581 to 1,033. However, germination tests showed that 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 >3,000 per 100 buds in each year. The mean viability of conidia associated with buds ranged from 0.7 to 1.9% and from 3.7 to 10.5% 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 ranged from 0.3 to 3.8% in the various treatments. Our results indicated that conidia were unlikely to overwinter on the surface of shoots or outer bud tissues, where they were exposed to fluctuating environmental conditions, and, consequently, 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.

Effects of atmospheric ammonia and ammonium sulphate on vesicular-arbuscular mycorrhizal colonization in three heathland species.

Three heathland species. Arnica montana L., Viola canina L. and Nardus stricta L., were exposed to gaseous ammonia and artificial rain containing ammonium sulphate for 11 wk. Plants infected with Glomus fasciculatum (Thaxter Sensu Gerdemann) Gerd & Trappe grew faster than non-mycorrhizal plants. Ammonia increased the shoot dry weight of both mycorrhizal and non-mycorrhizal V. Canina and mycorrhizal N. stricta. Only mycorrhizal V. canina and non-mycorrhizal N. stricta grew faster when supplied with increasing concentrations of ammonium sulphate. The nitrogen tissue concentration, in all three species, was not influenced by vesicular-arbuscular mycorrhiza (VAM), but increased when plants Wete treated with ammonia. Mycorrhizal and non-mycorrhizal N. stricta and mycorrhizal V. canina had a higher N tissue concentration when supplied with ammonium sulphate. The phosphorus tissue concentration was higher when plants were infected VAM, but tended to be lower in the presence of ammonia or ammonium sulphate. Mycorrhizal plants had higher total amounts of N and P than non-mycorrhizal plants and plants treated with ammonia had higher total amounts of N than non-treated plants. VAM colonization increased in all three species when treated with ammonia but was not affected by exposure to ammonium sulphate. Mycorrhiza decreased the sensitivity of N. stricta to drought, while ammonia decreased the sensitivity to drought for all three species. The effects of additional nitrogen in relation to N/P ratios are discussed with respect to the ecological consequences for these heathland species.

Throughfall below grassland canopies: a comparison of conventional and ion exchange methods.

Sampling of canopy fluxes (throughfall and stemflow) below low structured vegetation with a small-scale, intricate canopy architecture is difficult, and representative sampling with most methods is questionable. In the present study, two sampling methods for canopy fluxes below grassland vegetation are compared. Method I sampled canopy fluxes of moisture inefficiently, because stemflow volumes were not quantitatively included. Canopy fluxes of ions calculated with method I necessitated assumptions on equal concentrations in actually sampled throughfall and non-sampled stemflow. Method II sampled canopy fluxes of ions quantitatively, because the total volume of throughfall and stemflow percolated through a mixed bed of ion exchange resins below the canopy. Ion-specific differences between the two methods were observed. For ions with foliar leaching, such as K+ and Ca2+, higher canopy fluxes were recorded with method II than with method I. In contrast, for ions with foliar uptake, such as NH4+ and NO3-, canopy fluxes were found to be less with method II than with method I. Canopy fluxes of inorganic nitrogen below Mesobrometum grassland were 2.35 and 1.52 kmol(c) ha(-1) year(-1) for methods I and II, respectively, and 2.85 and 7.90 kmol(c) ha(-1) year(-1) for K+. It is argued that these differences result from under-estimated (foliar leaching) or over-estimated (foliar uptake) concentrations in stemflow by the first method. Canopy fluxes for SO4(2-) were not statistically different, indicating that canopy exchange of SOx was quantitatively unimportant, and that both methods estimated atmospheric input equally well.

Capture of atmospheric ammonium by grassland canopies.

Air pollution can cause a decline in species through acidification of the habitat. New data suggest that the decline may be due to eutrophication rather than acidification. In Western Europe, eutrophication largely results from atmospheric ammonium deposition. The amount deposited on vegetation is a function of its canopy structure. Deposition on grasslands has been underestimated, and a significant amount of the deposited ammonium appears to be assimilated by the plant canopy. These quantities are sufficient to initiate changes in the competitive relations among the plant species.