Richard Buchner

Inoculum Dynamics, Fruit Infection, and Development of Brown Rot in Prune Orchards in California

ABSTRACT Brown rot, caused by Monilinia fructicola, is a destructive disease of stone fruit in California. Disease management requires information on inoculum dynamics and development of latent and visible fruit infections during the season to help make decisions on timing of fungicide treatments and choice of cultural practices. In this study, the daily spore concentration (ascospores and conidia) of M. fructicola in the air was monitored with spore traps in two prune orchards during the growing seasons in 2001 and 2002. The spore concentrations were low to moderate at early bloom, increased at full bloom, and decreased to the lowest level at the end of bloom. Improper timing of fruit thinning and irrigation in midseason increased spore concentration in the air and fruit infections late in the season. Artificial fruit inoculations were conducted periodically in 10 prune orchards in 2002 and 2004, and incidence of fruit rot at different inoculation dates was assessed. Fruit rot development rate increased linearly with inoculation date during the growing season. Natural blossom and fruit infections were monitored periodically in 10 prune orchards, and incidence of latent fruit infection was determined by using the overnight freezing-incubation technique. Incidence of fruit rot also was assessed 2 weeks before harvest in these orchards. The incidence of latent fruit infection at the pit hardening stage significantly correlated with that at the late stages and with the incidence of fruit rot at harvest.

Tree water status and gas exchange in walnut under drought, high temperature and vapour pressure deficit

Summary Drought reduces photosynthesis in walnut (Juglans regia L.), but it is not known whether this is due mainly to the closure of stomata, or to possible effects on leaf biochemistry. In an attempt to answer this question we studied diurnal changes in the water status and gas exchange in droughted [50% crop evapotranspiration (ETc)] and fully irrigated (100% ETc) walnut trees, over 2 d. Stem water potential ( s) ranged from –0.5 MPa in the morning to –1.2 MPa in the afternoon under drought, and from –0.1 MPa to –0.4 MPa under full watering. Net CO2 assimilation (Amax) ranged from 15 µmol CO2 m–2 s–1 in the morning to 3 µmol CO2 m–2 s–1 in the afternoon under drought, and from 25 µmol CO2 m–2 s–1 in the morning to 10 µmol CO2 mm–2 s–1 in the afternoon under full watering. At these times, stomatal conductance (gs) varied from 0.2 to 0.02 mol H2O m–2 s–1 and from 0.7 to 0.2 mol H2O m–2 s–1, respectvely. Drought reduced the internal CO2 concentration (Ci) by about 55 µmol mol–1 on day-1, and by about 100 µmol mol–1 on day-2 and increased leaf temperature (Tl) by about 2°–5°C. The reductions in gs and Ci with drought suggest that lower photosynthesis was associated with stomatal closure. However, in each treatment, Amax decreased during the day, while Ci was stable, suggesting that photosynthesis was also reduced by a direct effect of heat on leaf biochemistry. Both Amax and gs correlated with Tl and with the leaf-to-air vapour pressure deficit (VPDl), but with different relationships for droughted and control trees. However, when stomatal limitations to photosynthesis were accounted for (i.e., based on the assumption that, under stomatal limitation, photosynthesis is proportional to Ci, a single relationship between Amax and Tl described all the data (R2 = 0.81). Thus, photosynthesis was limited by the closing of stomata under drought, and by a direct effect of heat on leaf biochemistry. These results suggest that hot and dry weather reduces photosynthesis and potential productivity in walnut in the absence of soil water deficit.

Colonization of Dormant Walnut Buds by Xanthomonas arboricola pv. juglandis Is Predictive of Subsequent Disease

The potential role of walnut buds as a driver of walnut blight disease, caused by Xanthomonas arboricola pv. juglandis, was addressed by quantifying its temporal dynamics in a large number of orchards in California. The abundance of X. arboricola pv. juglandis on individual dormant and developing buds and shoots of walnut trees varied by >10(6)-fold at any sample time and within a given tree. X. arboricola pv. juglandis population size in shoots was often no larger than that in the buds from which the shoots were derived but was strongly correlated with prior pathogen population sizes in buds. X. arboricola pv. juglandis populations on developing nuts were strongly related to that on the shoots on which they were borne. The incidence of disease of nuts in June was strongly correlated with the logarithm of the population size of X. arboricola pv. juglandis in dormant buds in March. Inoculum efficiency, the slope of this linear relationship, varied between years but was strongly related to the number of rain events following bud break in each year. Thus, inoculum of X. arboricola pv. juglandis present on dormant buds is the primary determinant of nut infections and the risk of disease can be predicted from both the numbers of X. arboricola pv. juglandis in buds and the incidence of early spring rain.

WALNUT BLIGHT MANAGEMENT USING XANTHOMONAS ARBORICOLA PV. JUGLANDIS DORMANT BUD POPULATION SAMPLING

Walnut blight orchard surveys have been underway for four years in Tehama County and three years in Butte County. In total, thirty walnut orchards are being monitored using dormant bud population assays for Xanthomonas arboricola pv juglandis (Xaj), individual orchard spray programs and the subsequent amount of walnut blight visually evaluated in early June. For the Tehama County orchards, bud populations are relatively low and effective spray programs are managing walnut blight damage. For Butte County orchards, walnut blight damage was high in 2012, however, lower blight disease pressure and effective spray programs in 2013 dramatically reduced walnut blight damage. Comparisons of the percent dormant buds infested with pathogen to the Xaj populations within buds revealed a very good relationship suggesting the percent infested buds can be used to indicate disease potential. For 2012 and 2013 the percent buds with pathogen did a good job of predicting disease risk. Typically, walnut orchards with walnut blight damage in the previous year are at risk of damage in the current year. Dormant bud population information appears to be a better predictor of disease risk. Relationships are surprisingly good particularly when spray programs and weather are uncontrolled variables. Walnut blight transects for the number of blighted walnuts per tree is beginning to show how much tree to tree variation exists within orchards. This information is valuable for determining how many trees are necessary for a representative sample.