D. P. Morgan

Aspergillus species and mycotoxins in figs from California orchards.

Although 23 different Aspergillus spp. decayed figs in California orchards, only A. niger occurred in more than 0.2% of the figs. The black-spored Aspergillus isolates that caused the disease fig smut were classified as A. niger var. niger, A. niger var. awamori, A. japonicus, and A. carbonarius. Different fungi differed in their association with Aspergillus Section Nigri (causal agents of fig smut) infections in figs. For example, most figs infected with Aspergillus Section Flavi (potential aflatoxin producers) also had infections by Section Nigri. For other fungi, there was either no significant relationship between fig infections by these fungi and Section Nigri or simultaneous infections by Section Nigri were fewer than expected. Insect damage to the fig fruit, predominantly by navel orangeworm (Amyelois transitella), did not significantly increase the colonization of figs by Aspergillus spp. The incidences of infection by Aspergillus (Sections Nigri, Aspergillus, Flavi, and Circumdati) in figs differed little for different harvests. Figs naturally infected with A. alliaceus, A. melleus, A. ochraceus, and A. sclerotiorum of Aspergillus Section Circumdati contained ochratoxin up to 9,600 ng/g, although only 40% of the figs with these fungi had more than a trace amount of ochratoxin. Aflatoxin contamination in figs naturally infected with Aspergillus Section Flavi varied according to the species involved. No aflatoxins were detected in all figs infected with A. tamarii and in most figs infected with A. flavus. High levels of aflatoxin (>100 ng/g) were detected in 83% of the figs infected by A. parasiticus, but in only 32% of the figs infected by A. flavus. Section Flavi isolates from fig orchard soils were tested for their ability to produce the mycotoxins aflatoxin and cyclopiazonic acid. Aspergillus parasiticus isolates always produced aflatoxin but never cyclopiazonic acid; A. flavus strain S (producers of small sclerotia) isolates always produced both aflatoxin and cyclopiazonic acid, but strain L (producers of large sclerotia) isolates frequently did not produce aflatoxin or cyclopiazonic acid; and A. tamarii isolates never produced aflatoxin but always produced cyclopiazonic acid. Aspergillus flavus was recovered from the soil, at fewer than 6 CFU/g of dry soil of every fig orchard assayed in 1992 and 1993. Although A. parasiticus was rarer in fig fruit than was A. flavus for each year, in orchard soil A. parasiticus was more frequent than A. flavus. Isolates of A. flavus strain L were much more common in the orchard soil and fig fruit than those of strain S. Figs in commercial orchards seem to be a favorable substrate for infection by and growth of Aspergillus spp.

Quantification of airborne spores of Monilinia fructicola in stone fruit orchards of California using real-time PCR

The fungal pathogen Monilinia fructicola causes blossom blight and fruit brown rot of stone fruits in California. In this study, spore densities in the air were monitored in six orchard/year combinations with Burkard spore traps. A real-time PCR assay was developed to efficiently quantify the dynamics of spore density in these orchards during the growing season. Different patterns of dynamics of spore density were observed in these orchards. A linear relationship between numbers of spores counted with a compound microscope and those determined with the real-time PCR assay was obtained, using the same samples of spore traps. Spore density in five of six orchard/year combinations ranged from 0.0 to 0.05 spores l−1, except for that in orchard 4, which showed much higher values of spore density in the air, as well as higher values and wider range of incidences of blossom infection and fruit rot than those in the other orchards. The results demonstrated a potential method to quantitatively determine spore inoculum potential in orchards by using a real-time PCR assay.

Risk Analysis of Brown Rot Blossom Blight of Prune Caused by Monilinia fructicola

Experiments under controlled environmental conditions were conducted during bloom of prune (Prunus domestica, L.) in 1999 and 2000 to assess the effects of inoculum concentration (IC), wetness duration (WD), temperature, and bloom stages on development of brown rot blossom blight of prunes. Branches from trees of a prune orchard were inoculated with Monilinia fructicola at different bloom stages and incubated at different temperatures with different periods of WD. The proportion of blighted blossoms (PBB) for each inoculated branch was determined. Bloom stage, IC, temperature, and WD significantly affected blossom blight of prunes. PBB at popcorn and full bloom stages was significantly greater than PBB at later bloom stages (P </=0.05). The optimal temperatures for blossom blight development were 22 to 26 degrees C, and Gaussian functions were used to describe the relationship between PBB and temperature. PBB linearly increased with increased IC. Linear regressions of PBB on WD were obtained for each combination of bloom stage, IC, and temperature. The parameters of these regressions were used in a computer program to produce the possible maximum PBB with 90% probability (PBB(90)) using stochastic simulations. Early bloom stages with a higher IC at temperatures from 20 to 25 degrees C were associated with more severe blossom blight than late stages with a lower IC at nonoptimal temperatures. Blossom blight did not occur at <10 or >30 degrees C and less than 4-h WD. However, longer than 4-h WD linearly increased incidence of blossom blight. A risk assessment table of blossom blight was produced for different environmental conditions to guide the control of prune brown rot.

Using incidence of Botrytis cinerea in kiwifruit sepals and receptacles to predict gray mold decay in storage

A field-monitoring system has been developed to predict the incidence in storage of gray mold decay of kiwifruit caused by Botrytis cinerea. Kiwifruits were harvested from nine vineyards in 1993 and 1994, and their sepals and receptacles (stem ends) were surface sterilized and placed on acidified potato dextrose agar. The incidence of B. cinerea was determined after incubating dishes at 6 to 7°C for 6 days followed by 3 days at 23°C. At commercial harvest time, kiwifruits were harvested from these vineyards and stored at a controlled-atmosphere, cold (-0.5°C) facility. Postharvest gray mold was recorded after 3 and 5 months of storage. In both years, the incidence of sepal colonization decreased 3 months after fruit set and then increased until harvest time. In contrast, the incidence of receptacle colonization increased continuously from 4 and I months after fruit set until harvest in 1993 and 1994, respectively. In 1993, the relationship between incidence of B. cinerea in fruit sepals or receptacles and incidence of gray mold after 3 and 5 months of fruit storage was significant, as determined by linear regression, for most sampling dates from 4 months after fruit set until harvest. In 1994, all regressions were significant (R 2 = 0.55 - 0.96, P 50%) incidence of sepal or receptacle colonization by B. cinerea distinguished (predicted) the majority of the vineyards as having low ( 6%) incidence of postharvest gray mold decay after 5 months of storage, respectively. In vineyards where the incidence of gray mold decay was low to moderate ( 6%) incidence of gray mold significantly reduced fruit decay in storage.

Dynamics and pattern of latent infection caused by Botryosphaeria dothidea on pistachio buds

A study was conducted in four commercial pistachio orchards to monitor the presence and pattern of external contamination and latent infections of buds by Botryosphaeria dothidea between 1998 and 2000. Symptomless buds were sampled every 2 to 3 weeks and analyzed either by a washing/crushing method or by direct plating of split (half) or intact buds on lactic acid potato dextrose agar. The proportion of infected buds varied among orchards over time. Levels of latent infections were highest in February and March when they reached as much as 60% in orchards of Glenn and Yolo counties. This period corresponded to the months with the highest rainfall. Buds collected from orchards in Glenn and Yolo counties had, in general, higher incidence of infection than buds from San Joaquin and Merced counties. Buds became infected in June immediately after their formation. Infection incidence on basal segments of buds from male trees was approximately twice (19%) that of apical or middle segments (11%). Plating of split buds resulted in similar levels of incidence as the plating of intact buds. The number of B. dothidea propagules on bud surfaces varied over time and among orchards. In general, the number of propagules per bud was highest in February and March (approximately seven propagules per bud) when the rainfall amount was highest.

Effects of latent infection, temperature, precipitation, and irrigation on panicle and shoot blight of pistachio in California

ABSTRACT Panicle and shoot blight, caused by a Fusicoccum sp., is an economically important disease of pistachio in California. Between 1999 and 2001, the disease severity was monitored throughout the growing season in 10 pistachio orchards, irrigated with drip, microsprinklers, low-angled (12 degrees ) sprinklers, or flood. The effect of temperature, precipitation pattern, irrigation system, and incidence of Fusicoccum sp. latent infection on panicle and shoot blight severity was quantified with a generalized linear model for repeated measures. The number of continuous rainy days in April and May and the cumulative daily mean temperatures from June to early September had a significant positive effect on panicle and shoot blight of pistachio leaves and fruit. Drip irrigation significantly decreased disease risk. Other factors, such as the number of discontinuous rainy days in April and May, the cumulative deviation from the 30-year average temperature during the dry days of April and May, the incidence of latent infection (only on leaves), and irrigation with microsprinklers or lowangled (12 degrees ) sprinklers were weak explanatory variables of panicle and shoot blight severity. Knowledge of panicle and shoot blight risk may contribute significantly to decisions regarding the appropriate application of fungicides, especially in years or fields of low risk.

First Report of Aspergillus Vine Canker of Table Grapes Caused by Aspergillus niger

A vine canker was first observed in the San Joaquin Valley, CA, in fall 1989, on exceptionally vigorous 1-year-old cv. Redglobe vines (Vitis vinifera) when vines were trained up the stakes. Since 1989, the same canker symptoms have been observed in Tulare, Kern, Fresno, and Riverside (Coachella Valley, CA) counties on cv. Redglobe, Crimson Seedless, Chardonnay, and Grenache vines. In affected vineyards, the disease resulted in the retraining of 2 to 6.1% of vines the following spring, using a shoot originating from below the canker. In a sample of 54 infected vines collected in 1997, 65% of cankers were found at the branching (crotch) of the vine, 24% along the shoot, or both (11%). All infections started through wounds caused by removing lateral shoots or leaves when the vine was topped to form cordons or possibly through growth cracks that occur on rapidly growing 1-year-old shoots. The first symptoms usually appear in August as red pinhead-size drops of sap on the surface of discolored tissue. By October to November, the canopies of vines girdled by the canker prematurely display fall colors and are very distinct from healthy vines. The trunk is slightly swollen and spongy where the canker occurs. Internal canker tissue is discolored and dead. Black spores are abundant within the canker, on the surface of the canker, or both. Callous tissue is often associated with the canker as the vine attempts to repair the damage with new tissue. Canker length can range from 3.5 to 26.5 cm (average 7.0 cm) and can affect the shoots cross section from 0.4 cm to completely girdling the shoot (up to 9.0 cm in circumference). Isolations from cankers or black sporulation inside the canker on acidified potato dextrose agar (APDA) consistently yielded Aspergillus niger van Tiegh. Six well-matured current-season canes of cv. Redglobe in an experimental vineyard at Kearney Agricultural Center were inoculated by inserting a 7-mm plug of mycelium from actively growing cultures on APDA in a cut made with a 7-mm cork borer or by brushing spores of the culture over the surface of six canes wounded with a sterile razor. Six canes were inoculated with a 7-mm plug of APDA and used as noninoculated controls. Inoculated sites were sealed with Parafilm to avoid dehydration. Inoculation of grapevines with A. niger resulted in cankers similar to those observed in commercial vineyards 5 months after inoculation. Cankers ranged from 2.4 to 4.2 cm for mycelial-plug inoculation (100% of canes infected) and 2.3 to 7.3 cm for spore-brushing inoculation (67% infected). Noninoculated control canes were not infected. In another experiment, inoculation of 10 canes each with A. niger on 17 May, 10 June, 2 July, 21 July, and 16 August resulted in 50, 60, 90, 90, and 100% canker formation, respectively, 5 to 8 months after inoculation, suggesting summer inoculations were more effective than spring inoculations. Reisolation from infected canes on APDA revealed A. niger. Aspergillus species in section Nigri have been reported to be among the pathogens involved in the bunch rot complex (1,2), but to our knowledge, this is the first report of A. niger causing a serious canker of vigorously growing grape vines. References: (1) W. B. Hewitt. Berry rots and raisin molds. Pages 26-28 in: Compendium of Grape Diseases. R. C. Pearson and A. C. Gohen, eds. The American Phytopathological Society, St. Paul, MN, 1994. (2) W. R. Jarvis and J. A. Traquair. Plant Dis. 68:718, 1984.

First Report of Botryosphaeria rhodina Causing Shoot Blight of Pistachio in California

In the summers of 2000 and 2001, shoot blight was observed in pistachios (Pistacia vera L.) grown in Kern County, California. Black, necrotic lesions developed at the base of shoots originating from contaminated or partially infected buds. Infection moved upward resulting in a progressive wilting and blighting of leaves. Leaf blades on infected shoots withered, and petioles became necrotic. Symptoms have been considered characteristic of infection by Botryosphaeria dothidea (Moug.:Fr.) Ces. & de Not., but this pathogen causes panicle and shoot blight of pistachio (1). However, there were no symptoms of any fruit panicle infections on trees we observed. Isolations on acidified potato dextrose agar from the base of blighted shoots in both years revealed a fast-growing fungus producing pycnidia which was identified as the anamorph Lasiodiplodia theobromae (Pat.) Griffon & Maubl. of B. rhodina Berk. & Curt. Arx. Identification of the pathogen was based on characteristic dark brown, oval pycnidiospores with striations on the surface of the spore along the long axis. Pathogenicity tests were performed on 12 Kerman pistachio trees grown at Kearney Agricultural Center, in Parlier, CA, using three isolates recovered from pistachios grown in two locations. Six to 16 current season shoots of pistachio trees (1 to 2 shoots per tree) were wounded with a 5-mm-diameter cork borer, and a mycelial plug of 5-day-old cultures of B. rhodina was inserted in each wound. Shoots were wrapped with Parafilm to prevent desiccation of inoculum. Six other shoots (one per tree) were inoculated similarly with mycelial agar plugs of a pistachio isolate of B. dothidea and served as positive controls, while six similar shoots were inoculated with only agar plugs and served as negative controls. Wilting of lower leaves in the majority of inoculated shoots started within 4 days for B. rhodina and 7 days for B. dothidea. Depending on the isolate of B. rhodina, 1 to 5 shoots and 50 to 80% of leaves were blighted within 7 days after inoculation. All inoculated shoots were left on the trees until 3 to 4 months after inoculation, pruned and assessed again. For inoculations done in September 2001, 33 to 71% of shoots were blighted, and the rest had cankers ranging from 22.5 to 28 mm long and 13.5 to 23.5 mm wide. A majority (67 to 100%) of shoots had pycnidia of the pathogen present. For inoculations done in October 2001, none of the shoots was blighted, but cankers ranged from 5 to 55.4 mm long and 6 to 22 mm wide and 33.3 to 100% developed pycnidia. B. rhodina was isolated from all inoculated shoots but not from negative controls or those inoculated with B. dothidea. Inoculations of shoots with B. dothidea produced similar symptoms as those of B. rhodina. Shoots that served as negative controls did not develop symptoms. Because panicle and shoot blight of pistachio caused by B. dothidea has developed to epidemic levels in commercial pistachio orchards and is of concern to the pistachio industry in California, it would be of interest to monitor how much shoot blight caused by B. rhodina would eventually develop over the years in commercial pistachio orchards. A survey was initiated in 2002 to determine how widespread B. rhodina is in California pistachios. To our knowledge, this is the first report worldwide of B. rhodina causing shoot blight of pistachio. Reference: (1) T. Michailides. Panicle and shoot blight. Page 68 in: Compendium of Nut Crop Diseases in Temperate Zones. B. L. Teviotdale, T. J. Michailides, and J. W. Pscheidt, eds. American Phytopathological Society, St. Paul, MN 2002.

Wind scab of french prune : symptomatology and predisposition to preharvest and postharvest fungal decay

Severe scabbing, termed wind scab (WS), of French prune (Prunus domestica «French») was caused by developing fruit rubbing against other fruit, leaves, and shoots during strong wind gusts prevailing from north to northwest. WS occurred only during years in which north or northwest winds exceeded 20 km/hr for at least 10 days within 3 wk after full bloom. The affected areas developed several layers of cutinized cells, the outer layers showing deep fractures that retained moisture and facilitated germination of fungal spores and penetration by their germ tubes (.)

Effects of Wound Size, Amount of Sap, and Number of Blighted Nuts on Infection of Pistachio Organs by Neofusicoccum mediterraneum

Laboratory and field studies were conducted to determine the effects of wounding of nut exocarp, susceptibility period after wounding, and sap nut on infection of pistachio nut by Neofusicoccum mediterraneum, the main causal agent of panicle and shoot blight of pistachio. Under controlled conditions and in the field, detached nuts were inoculated with a conidial suspension 30 min before or after wounding. In addition, a 30-µl drop of pistachio sap was placed on the surface of noninjured nuts 30 min before or after they were wounded and then inoculated. Wounding increased the disease severity under both controlled and field conditions. The addition of sap increased the susceptibility of nuts under controlled conditions but not in the field, possibly due to dried sap blocking the pathogen infection. When nuts of Kerman, Kalehghouchi, and Golden Hills pistachio were wounded and inoculated at different time periods after wounding; the nuts of the three cultivars were highly susceptible to pathogen infection during at least the first 24 h after wounding. Under field conditions, there was not a clear effect of increasing the number of inoculated nuts per panicle or the inoculation position (basal or apical) in killing (blight) of the panicle. Conversely, inoculations conducted with mycelial plugs resulted in higher disease, increased the proportion of dead panicles, and resulted in faster symptom expression than inoculations conducted with a conidial suspension. To determine the temporal infection pattern, leaves and panicles were regularly collected from different orchards from 2004 to 2007 and the pathogen was isolated on medium. Important differences in latent infection were detected between years and orchards, with nut and rachis being, in general, the tissues most susceptible to infection. Results of this study help in better understanding the dynamic of infection and colonization of pistachio by N. mediterraneum.