Discharge and Dissemination of Ascospores by Venturia inaequalis During Dew

Abundant airborne ascospores of the apple scab pathogen (Venturia inaequalis) have never before been observed during periods of dew. We studied ascospore release in V. inaequalis in two orchards in southeastern Norway using Burkard 7-day volumetric spore traps. At Ås in 1990, 1992, and 1997, and at Svelvik in 1992, a total of 14.8, 1.4, 0.27, and 26.9%, respectively, of the season's total spore release was trapped during periods of dew. Dew followed by spore release was observed 22 days at the two locations. During one night with dew at Ås in 1990 and two nights with dew at Svelvik in 1992, approximately 13 and 20%, respectively, of the season's total spore numbers were observed. High numbers of spores were trapped prior to sunrise, and on an average, 48.4% of the spores were trapped prior to 0400 in the morning. Episodes in which more than 1% of the season's inoculum was released during dew occurred around bloom of apple, which is the peak period for ascospore discharge, and followed more than 2 days of fair weather (clear, warm days and cool, humid nights). The ordinary suppression of ascospore release in V. inaequalis during darkness has been overcome in previous studies under laboratory conditions when protracted periods favorable for ascospore maturity occur without opportunity for ascospore discharge. This is the first confirmed report of relatively large (>10% of the season's total inoculum) numbers of airborne ascospores in orchards during dew. The sequential occurrence of specific weather conditions, for example (i) fair-weather days, (ii) cool nights with abundant dew formation, (iii) significant release and dispersal of airborne ascospores, and (iv) poor drying conditions or additional hours of leaf wetness due to fog or rain, would be required for dew-released ascospores to constitute a threat of infection. Absent the foregoing, release during dew is more likely to deplete the ascospore supply with no consequent increase in the overall risk of disease.

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A New Phytosanitary Method to Reduce the Ascospore Potential of Venturia inaequalis

Plant Disease ◽

10.1094/pdis-07-16-0994-re ◽

2017 ◽

Vol 101 (3) ◽

pp. 414-420 ◽

Cited By ~ 1

Author(s):

Franziska M. Porsche ◽

Barbara Pfeiffer ◽

Andreas Kollar

Keyword(s):

Leaf Litter ◽

Yeast Extract ◽

Untreated Control ◽

Venturia Inaequalis ◽

Apple Scab ◽

Nutrient Media ◽

Apple Leaves ◽

Litter Decay ◽

Ascospore Discharge ◽

Infection Pressure

Ascospores of Venturia inaequalis, released from pseudothecia in overwintered, infected apple leaves, serve as the primary inoculum for apple scab. In this study, we tested a new sanitation strategy to reduce ascospore inoculum under orchard conditions over three overwintering periods. After leaf fall, nutrient media containing different concentrations of degraded casein or a yeast extract from Saccharomyces cerivisiae were applied to leaf litter infected with apple scab. The application of 30 and 60% yeast extract showed the greatest efficacy, and significantly reduced ascospore discharge by 99% (P < 0.01) in 2013 and 2014. The efficacy of the treatments did not differ from treatment with 5% urea (P > 0.05). Leaf litter decay was accelerated in the plots treated with yeast extract compared with untreated control plots. Moreover, apple leaves treated with yeast extract had completely decayed due to earthworm activity before ascospore maturity. In comparison, up to 26% of the leaves in untreated control plots had not decayed. These results suggest that the treatment of leaf litter with yeast extract can almost completely eliminate apple scab inoculum in the course of the whole primary season. These sanitation practices may be beneficial for both organic and conventional cultivation. The reduced infection pressure may allow growers the usage of fungicides with lower efficacy or to reduce the number of applications needed to manage apple scab in spring.

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A Comparison of Methods Used to Estimate the Maturity and Release of Ascospores of Venturia inaequalis

Plant Disease ◽

10.1094/pdis.2004.88.8.869 ◽

2004 ◽

Vol 88 (8) ◽

pp. 869-874 ◽

Cited By ~ 15

Author(s):

David M. Gadoury ◽

Robert C. Seem ◽

William E. MacHardy ◽

Wayne F. Wilcox ◽

David A. Rosenberger ◽

...

Keyword(s):

Venturia Inaequalis ◽

Infected Leaf ◽

Degree Days ◽

Site Specific ◽

Degree Day ◽

Ascospore Discharge ◽

Ascospore Development ◽

Ascospore Release ◽

The Mean ◽

Highly Correlated

Maturation and release of ascospores of Venturia inaequalis were assessed at Geneva and Highland, NY, and at Durham, NH, by microscopic examination of crushed pseudothecia excised from infected apple leaves that were collected weekly from orchards (squash mounts) in 14 siteyear combinations. Airborne ascospore dose was monitored at each location in each year of the study by volumetric spore traps. Additional laboratory assessments were made at Geneva to quantify release from infected leaf segments upon wetting (discharge tests). Finally, ascospore maturity was estimated for each location using a degree-day model developed in an earlier study. Ascospore maturation and release determined by squash mounts and discharge tests lagged significantly behind cumulative ascospore release as measured by volumetric spore traps in the field. The mean date of 98% ascospore discharge as determined by squash mounts or discharge tests occurred from 23 to 28 days after the mean date on which 98% cumulative ascospore release had been detected by volumetric traps. In contrast, cumulative ascospore maturity estimated by the degree-day model was highly correlated (r2 = 0.82) with observed cumulative ascospore release as monitored by the volumetric traps. Although large differences between predicted maturity and observed discharge were common during the exponential phase of ascospore development, the date of 98% cumulative ascospore maturity predicted by the model was generally within 1 to 9 calendar days of the date of 98% cumulative ascospore recovery in the volumetric traps. Cumulative ascospore discharge as monitored by the volumetric traps always exceeded 98% at 600 degree days (base = 0°C) after green tip. Estimating the relative quantity of primary inoculum indirectly by means of a degree-day model was more closely aligned with observed ascospore release, as measured by volumetric traps, than actual assessments of ascospore maturity and discharge obtained through squash mounts and discharge tests. The degree-day model, therefore, may be a more accurate predictor of ascospore depletion than squash mounts or discharged tests, and has the added advantage that it can be widely applied to generate site-specific estimates of ascospore maturity for any location where daily temperature data are available.

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Use of a Rainfall Frequency Threshold to Adjust a Degree-Day Model of Ascospore Maturity of Venturia inaequalis

Plant Disease ◽

10.1094/pd-89-0198 ◽

2005 ◽

Vol 89 (2) ◽

pp. 198-202 ◽

Cited By ~ 20

Author(s):

Arne Stensvand ◽

Håvard Eikemo ◽

David M. Gadoury ◽

Robert C. Seem

Keyword(s):

New Hampshire ◽

Venturia Inaequalis ◽

Simple Modification ◽

Degree Day ◽

Rainfall Frequency ◽

Spore Release ◽

Ascospore Release ◽

Frequency Threshold ◽

Rain Events ◽

Southern Norway

Estimates of ascospore maturity generated by a model developed previously in New Hampshire, United States, were compared with the cumulative release of ascospores in southern Norway as monitored by volumetric spore traps at one site for 12 years, and at two additional sites for 2 years. In locations and years with frequent rain events, model-estimated ascospore maturity closely approximated observed ascospore release. However, in years with protracted dry periods of 1 to 3 weeks with no or little rain, not only was spore release delayed, but release continued to lag behind predicted maturity even after several rain events subsequent to the dry interval. By halting degree-day (base = 0°C) accumulation if 7 consecutive days without rain occurred, accuracy of the model during “dry” years was greatly improved, without substantially affecting accuracy in “wet” seasons. With minimal additional effort on the part of the user, this simple modification increases the accuracy of model-derived estimates of ascospore maturity when lack of rain slows ascospore maturation.

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Dynamics of Ascospore Release by Apiosporina morbosa from Sour Cherry Black Knots

Plant Disease ◽

10.1094/pdis.2000.84.1.45 ◽

2000 ◽

Vol 84 (1) ◽

pp. 45-48 ◽

Cited By ~ 7

Author(s):

W. McFadden-Smith ◽

J. Northover ◽

W. Sears

Keyword(s):

Regression Model ◽

Environmental Conditions ◽

Sour Cherry ◽

Peak Period ◽

Ascospore Discharge ◽

Ascospore Release ◽

The Relationship ◽

Cherry Trees

Ascospore discharge was monitored from black knots excised from sour cherry trees in May 1992 and 1993, and March, April, and May 1994 and 1995. Rain, temperature, and wetness duration were monitored and a regression model was developed to describe the relationship between environmental conditions and ascospore discharge. Regardless of the date of excision, the peak period for ascospore release occurred from late May (shuck split) through the end of June (shoots 20 cm long), considerably later than previously reported from knots on plum. Fewer ascospores were released from knots excised in early March compared to later dates in 1994. In 1995, the spring was much warmer than in 1994 and ascospore release from March-excised knots was comparable to that from knots excised later in the spring. The period of release and total number of ascospores released was similar from non-excised knots and from knots excised in April or May 1994. Release of ascospores was dependent on rainfall and temperature, but not on the duration of wetness.

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A new apple scab risk model that integrates ascospore release infection risk and susceptible leaf area

New Zealand Plant Protection ◽

10.30843/nzpp.2004.57.6935 ◽

2004 ◽

Vol 57 ◽

pp. 20-24 ◽

Cited By ~ 3

Author(s):

R.M Beresford ◽

W.R. Henshall ◽

J.W. Palmer

Keyword(s):

Leaf Area ◽

Risk Model ◽

Venturia Inaequalis ◽

Apple Scab ◽

Black Spot ◽

Infection Risk ◽

Weather Data ◽

Scab Control ◽

Ascospore Release ◽

Maturation Rate

A new model has been developed for assessing daytoday variation in risk of infection of apples by Venturia inaequalis the scab or black spot pathogen The model comprises three components ascospore availability wetnessbased infection risk based on Mills periods and susceptible leaf area The ascospore and wetnessbased infection risk components were adapted from previous models whereas the susceptible leaf area component is new When the model used weather data from Hawkes Bay and Nelson in spring 2003 the predicted risk incidence was determined mostly by wetnessbased infection risk but the magnitude of risk periods was greatly influenced by predicted ascospore release The susceptible leaf area component predicted a hitherto unidentified increase in infection risk after the peak in ascospore maturation rate had occurred The model is intended to assist in fungicide selection and timing for scab control in New Zealand apples but needs to be field tested before implementation

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EFFECTS OF SEASONAL FUNGICIDE SPRAYS ON PERITHECIUM FORMATION AND ASCOSPORE PRODUCTION IN VENTURIA INAEQUALIS

Canadian Journal of Plant Science ◽

10.4141/cjps75-115 ◽

1975 ◽

Vol 55 (3) ◽

pp. 737-742 ◽

Cited By ~ 4

Author(s):

R. G. ROSS ◽

R. J. NEWBERY

Keyword(s):

Venturia Inaequalis ◽

Apple Scab ◽

Malus Pumila ◽

In Vitro Production ◽

Ascospore Discharge ◽

Malus Pumila Mill ◽

Terminal Shoot ◽

Vitro Production ◽

Ascospore Production

Seasonal sprays of the systemic fungicides benomyl, thiophanate-methyl, Bay Dam 18654 and Bavistin applied for the control of apple scab, Venturia inaequalis (Cke.) Wint., either completely or almost completely suppressed the production of ascospores in apple (Malus pumila Mill.) leaves overwintered in the orchard. In vitro production of perithecia on leaves sterilized with propylene oxide was also inhibited by these sprays, although some treatments suppressed ascospore discharge in overwintered leaves that failed to inhibit perithecium formation on the sterile leaves. Ascospore production was also reduced in leaves from spur clusters that had been sprayed with Dikar and metiram. In terminal shoot leaves from Dikar-sprayed trees, ascospore production was higher in leaves collected from near the tip than in leaves collected from near the base of the terminal growth.

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The effectiveness of potassium phosphite and captan mixture in controlling Venturia inaequalis in apple orchards

Pesticidi i fitomedicina ◽

10.2298/pif1402137a ◽

2014 ◽

Vol 29 (2) ◽

pp. 137-143

Author(s):

Goran Aleksic ◽

Tatjana Popovic ◽

Mira Starovic ◽

Slobodan Kuzmanovic ◽

Nenad Dolovac ◽

...

Keyword(s):

Venturia Inaequalis ◽

Apple Scab ◽

Weather Conditions ◽

Meteorological Conditions ◽

Environmental Damage ◽

Weather Station ◽

Apple Orchards ◽

High Efficacy ◽

Potassium Phosphite ◽

Leaf Infection

Venturia inaequalis is a common apple disease in Serbia and its intensity depends on weather conditions. The most successful method of combating apple scab is to use a lower dose of an active ingredient and increase its effectiveness by adding substances that do not cause environmental damage. The effectiveness of a fungicide mixture of captan and potassium phosphite against V. inaequalis was investigated in apple orchards in 2008 and 2009. A relevant EPPO standard method was used in all trials. Captan 50 WP was used as a standard fungicide. In order to determine the exact conditions marking the onset of apple infection, key meteorological conditions were monitored in the orchards using an automated iMetos weather station and a Lufft electronic instrument. The results showed high efficacy of the tested fungicides which remained similar in both years of investigation. The intensity of leaf infection after treatment with the mixture of captan and potassium phosphite was 0.7-2.4%, while fruit infection was 0.5-3.0%. The efficacy of this mixture ranged from 96.3-97.9% on leaves and 87.4-98.7% on fruits. The standard fungicide Captan 50 WP showed an efficacy ranging from 95.3-96.7% on leaves and from 87.8-99.3% on fruits. No statistically significant differences were found between the fungicide tested and the standard fungicide.

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Ascospore Release and Infection of Apple Leaves by Conidia and Ascospores of Venturia inaequalis at Low Temperatures

Phytopathology ◽

10.1094/phyto.1997.87.10.1046 ◽

1997 ◽

Vol 87 (10) ◽

pp. 1046-1053 ◽

Cited By ~ 35

Author(s):

Arne Stensvand ◽

David M. Gadoury ◽

Terje Amundsen ◽

Lars Semb ◽

Robert C. Seem

Keyword(s):

Low Temperatures ◽

Venturia Inaequalis ◽

Field Studies ◽

Leaf Wetness ◽

Apple Trees ◽

Infection Period ◽

Apple Leaves ◽

Ascospore Discharge ◽

Ascospore Release ◽

The Mean

Mills' infection period table describes the number of hours of continuous leaf wetness required at temperatures from 6 to 25°C for infection of apple leaves by ascospores of Venturia inaequalis and reports that conidia require approximately two-thirds the duration of leaf wetness required by ascospores at any given temperature. Mills' table also provides a general guideline that more than 2 days of wetting is required for leaf infection by ascospores below 6°C. Although the table is widely used, infection times shorter than those in the table have been reported in lab and field studies. In 1989 a published revision of the table eliminated a potential source of error, the delay of ascospore release until dawn when rain begins at night, and shortened the times reported by Mills for ascospore infection by 3 h at all temperatures. Data to support the infection times below 6°C were lacking, however. Our objective was to quantify the effects of low temperatures on ascospore discharge, ascospore infection, and infection by conidia. In two of three experiments at 1°C, the initial release of ascospores occurred after 131 and 153 min. In the third experiment at 1°C, no ascospores were detected during the first 6 h. The mean time required to exceed a cumulative catch of 1% was 143 min at 2°C, 67 min at 4°C, 56 min at 6°C, and 40 min at 8°C. At 4, 6, and 8°C, the mean times required to exceed a cumulative catch of 5% were 103, 84, and 53 min, respectively. Infection of potted apple trees by ascospores at 2, 4, 6, and 8°C required 35, 28, 18, and 13 h, respectively; substantially shorter times than previously were reported. In parallel inoculations of potted apple trees, conidia required approximately the same periods of leaf wetness as ascospores at temperatures from 2 to 8°C, rather than the shorter times reported by Mills or the longer times reported in the revision of the Mills table. We propose the following revisions to infection period tables: (i) shorter minimum infection times for ascospores and conidia at or below 8°C, and (ii) because both ascospores and conidia are often present simultaneously during the season of ascospore production and the required minimum infection times appear to be similar for both spore types, the adoption of a uniform set of criteria for ascosporic and conidial infection based on times required for infection by ascospores to be applied during the period prior to the exhaustion of the ascospore supply. Further revisions of infection times for ascospores may be warranted in view of the delay of ascospore discharge and the reduction of airborne ascospore doses at temperatures at or below 2°C.

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Role of organosilicone surfactant in apple scab control under scab conducive weather conditions

International Journal of Horticultural Science ◽

10.31421/ijhs/14/3/797 ◽

2008 ◽

Vol 14 (3) ◽

Author(s):

I. J. Holb

Keyword(s):

Untreated Control ◽

Venturia Inaequalis ◽

Apple Scab ◽

Weather Conditions ◽

Fruit Production ◽

Application Of Surfactants ◽

Fungus Infection ◽

Scab Control ◽

Integrated Fruit Production ◽

Organosilicone Surfactant

Fungicides of integrated fruit production (dithianon, captan, and diclofluanid) and an organosilicone surfactant were compared in spray schedules from green tip until summer in order to control apple scab caused by Venturia inaequalis and to evaluate their phytotoxicity on fruit. Sixteen sprays of 1.8 kg ha-1captan, 0.41 ha-1 dithianon, and 1.8 kg ha-1 diclolfluanid significantly (P=0.05) reduced the incidence of leaf or fruit scab compared to unsprayed products. All fungicides applied with organosilicone at 0.1% resulted in lower incidence of scab on young and older leaves as well as on harvested fruit, but these were not statistically always better than fungicides applied alone. In case of diclofluanid, the fungicide applied with organosilicone at 0.1% resulted in significantly lower (P = 0.001) incidence of scab on young and older leaves. Diclofluanid applied with organosilicone at 0.1% gave the best scab control on leaf and fruit. Treatments applied with fungicides alone had no significant effect on plant phytotoxicity compared to untreated control. All fungicides applied with organosilicone at 0.1% increased (P = 0.05) fruit damage compared to untreated control or fungicide applied alone, though these were not always significantly different. In case of percentage of fruit russet, treatment of dithianon 0.4 1/ha + 0.1% organosilicone significantly increased fruit russet, while fruit russet index significantly increased in the treatment of diclofluanid 1.8 kg/ha + 0.1% organosilicone compared to untreated control. In sum, application of surfactants can help to increase efficacy of scab fungicides; and consequently, to reduce the risk of fungus infection under high scab disease pressure. This fact may also be helpful in fungicide resistance management and reduced-spray programs with accurate scab warning systems.

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