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Placing Estimates of Ascospore Maturity in Context Introduction - The percentage of the season's spores that have matured is only part of the story. The stage of bud development, scab levels at the end of the previous year, and current weather conditions will often outweigh relative ascospore maturity in determining the risk of disease. Estimates provided by the Ascospore Maturity Model are one of many useful indicators of how the risk of apple scab changes during the first 6-8 weeks after bud break. The apple scab pathogen (Venturia inaequalis) overwinters in fallen infected leaves as small spherical fruiting bodies called pseudothecia, which ripen in spring and discharge ascospores during rain events. Ascospores are the principal source of primary inoculum for apple scab in the northeastern US. In severely diseased orchards, conidia within dormant buds may provide additional inoculum that will supplement the supply of ascospores. The actual number of infections resulting from any rain event is related to several interacting factors. These are described below. Target size - The targets for airborne ascospores are leaves and fruit. Apple fruit buds pass through a succession of growth stages during the first 8 weeks of the growing season. The generally recognized stages are green tip, 1/2-inch green, tight cluster, pink, bloom, petal fall, and fruit set. The probability of a spore contacting the target is a function of the target size.    During rain, ascospores are discharged only a few hundredths of an inch above the surface of overwintered leaves on the orchard floor. Thereafter, ascospores are carried aloft by turbulence, and move through tree canopies on roughly horizontal trajectories until they either settle to the ground, contact the tree, or leave the orchard. Initial target size is 0 in dormant buds. Target size, or surface area, increases slowly between green tip and tight cluster (Fig. 1A), and then increases exponentially between tight cluster and petal fall (Fig. 1A). Why focus on fruit buds? Because fruit buds predominate on modern spur-type trees during the primary infection season for apple scab. Vegetative shoot development begins up to 2 weeks later than fruit buds, and bourse shoot extension does not accelerate until after bloom. Target susceptibility - The susceptibility of leaves decreases rapidly with age as they acquire what is known as ontogenic resistance. Susceptibility decreases exponentially between 1/2-inch (1 cm) green and bloom (Fig. 1B). So, as leaves become bigger targets, they are also becoming less susceptible to infection. These two contradicting processes largely determine the probability that an ascospore will both land upon and successfully infect a fruit cluster during a favorable rain event.  Interactions of leaf size, leaf susceptibility, and ascospore maturity - Airborne ascospores are rarely detected before emergence of leaves from buds. The maximum density of airborne ascospores generally occurs near the tight cluster stage of bud developmentThe supply of ascospores is usually depleted shortly after bloom. In most years, the prevalence of airborne ascospores is well-described by the Ascospore Maturity Model. Figure 1C shows a distribution of airborne ascospore dose recorded at Geneva, NY. As in Figures 1A and 1B above, the distribution is on a 0 to 1 scale. Figures 1A-C are useful to illustrate how target size, target susceptibility and ascospore availability interact to influence primary disease pressure at different stages of bud development (Fig. 1D). We call this the "Relative Risk of Infection", or RRI, and again we have expressed it as a value between 0 and 1. On any given day, the risk of primary (ascosporic) infection can be calculated as the product of target size, target susceptibility, and ascospore maturity. Note well that the risk curve in Fig 1D refers only to infection by ascospores, and is valid only if ascosporic infection is currently well-controlled. Hopefully, this is the case in most commercial orchards during the period from green tip to petal fall. RRI is not relevant to control actions if scab lesions are present above trace levels between green tip and petal fall. Conidia produced on foliar lesions at such early growth stages pose a severe threat to fruit, irrespective of the value of RRI, and intensive spraying will be required to prevent development of severe fruit scab. Certain features of RRI are noteworthy. First, dramatic changes in RRI occur over periods of only a few days. The magnitude of these changes might be overlooked if only one component of RRI is being considered. Second, the distribution of RRI (Fig. 1D) is more narrow than for any of the component variables (Fig. 1A-C). How is RRI relevant to controlling scab? It illustrates the critical impact that one or two well-timed fungicide sprays may have. More than 90% of the total reduction of scab observed from the entire spray program usually comes from the one or two fungicide applications centered on that peak in the distribution of RRI (roughly tight cluster to early bloom). It's not that reducing the other 10% of the risk is unimportant, but it's rather meaningless if mistakes are made during the critical period Values of RRI are often highly correlated with actual disease development. Figure 2 shows the distribution of RRI in a McIntosh research orchard. The distribution of RRI closely overlays the distribution of lesions that developed during infection periods at each stage of tree development (Fig. 2). However, note well that RRI describes only the relative changes from day to day in the risk of scab infection. Absolute risk is largely determined by the level of disease that occurred the previous year. This is discussed in the next section.  Epidemics will begin earlier in orchards with large overwintering populations of the pathogen than in relatively "clean" orchards. An estimate of the potential number of ascospores that could be produced per square meter of orchard floor (Potential Ascospore Dose, or PAD) can be used to quantify just how dirty or clean an orchard is. Orchards with a PAD of less than 600 ascospores/m2/yr generally don't require protection for apple scab before the tight cluster stage (roughly the time when RRI reaches a maximum). PAD (the previous season's level of scab) sets the absolute mark from which RRI (relative risk of infection) fluctuates. A PAD of 10,000 ascospores per m2 (very high) and an RRI of 0.05 (very low) can produce the same number of infections as a PAD of 1000 and an RRI of 0.5. The relationship is pretty straightforward. Lots of spores can produce lots of lesions. For this reason, we recommend that you always consider PAD, or some comparable measure of the previous season's scab level when interpreting the estimates of the Ascospore Maturity Model. A small percentage of mature ascospores is still a very large number of ascospores when PAD is high, and can present a serious risk of infection if trees are not adequately protected. PDF file-Original PAD research Latent infections, fungistasis, and late-season apple scab - A variety of unusual leaf symptoms sometimes occur on fungicide-sprayed trees, ranging from extensive underleaf scab, to small red dots, to completely symptomless leaves; all harboring actively sporulating colonies of the apple scab pathogen. These symptoms occur more frequently in orchards with reduced spray programs, where post-infection applications are made, and where DMI fungicides are used against scab. In such situations, scab can build up on older leaves, which often do not express typical symptoms of apple scab. The apple scab pathogen can survive beneath the cuticle of leaves treated with DMI fungicides. When DMI fungicides are applied post-infection or at extended intervals (resulting in post-infection activity by default), leaves often develop chlorotic spots assumed to be "eradicated" infections. However, during the course of the summer, we have found that approximately 50% of these chlorotic spots eventually progress to become sporulating lesions when spraying is discontinued (Fig. 4),. Many of these suppressed lesions give rise to pseudothecia, which overwinter and mature normally and release abundant ascospores the following spring. Therefore, the threat posed by suppressed infections is not limited to late-season development of scab, but includes the cyclical buildup of inoculum during overwintering, and the underestimation of PAD (Potential Ascospore Dose in the spring. What happened: its bloom and I already have severe scab! - TThere is no mystery concerning what is required for severe disease. If susceptible host tissue is present and it is raining and it is sufficiently warm: severe disease requires abundant inoculum. The highest levels of airborne ascospores ever reported occurred in 1983 in a research orchard in New Hampshire. At this orchard in 1982, approximately 20% of the trees were unsprayed the previous year and were nearly defoliated by apple scab. At green tip in 1983, the cumulative airborne dose was 1,758 ascospores/cubic meter and by 1 cm (half-inch) green the cumulative dose had reached 45,719 ascospore/cubic meter. Rain events were both frequent and long during the month following bud break. What level of cluster infection was attributable to the above density of ascospores on unsprayed trees? Each cluster bore an average of 1.2 visible lesions when the first production of secondary spores (conidia) was observed at bloom. The bottom line: if you see severe levels of disease at bloom, you had both a large overwintering population of the pathogen (high PAD) and an ineffective spray program to date. What about abandoned orchards? - Both practical and theoretical studies support the contention that scab in most orchards arises from ascospores produced within the orchard, rather than from outside sources. Large area sources, such as abandoned orchards, could conceivably contribute to PAD in a commercial orchard. Unfortunately, we simply do not know enough about the strength of these sources, or the loss of ascospores from the air as distance from the source increases, to definitively state when they pose no risk. What we can say is that with the exception of cases in which abandoned orchards physically abut commercial orchards, there has been no demonstrated connection between proximity to such sources and scab severity in commercial orchards. That's good news if you don't abut an abandoned orchard. What happens in your orchard is largely under your control. Overintering of the scab pathogen in dormant infected buds - Conidia of V. inaequalis are capable of surviving winter at a low frequency within buds in New York.. However, because detectable levels of bud infestation by conidia apparently requires severe leaf scab, bud infestation is likely to occur only in orchards that also have abundant ascosporic inoculum. In the final analysis, if there was enough scab last year to generate bud infections, the orchard has a dangerously high inoculum dose and should be sprayed intensively no matter where the inoculum spent winter. PDF file- Original bud infection research For more information on how to interpret the model estimates or general information on epidemiology and control of scab, click on the topics below.
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