David P. Morgan

Significance of thinned fruit as a source of the secondary inoculum of Monilinia fructicola in California nectarine orchards

The significance of thinned fruit as a source of secondary inoculum in the spread of brown rot, caused by Monilinia fructicola, under semi-arid weather conditions of the San Joaquin Valley in California, was investigated in seven nectarine orchards in 1995 and 1996. Between 6 and 60% (depending on the orchard) of thinned fruit showed sporulation by M. fructicola. Brown rot was significantly less severe at preharvest (five orchards) and postharvest (one orchard) on fruit harvested from trees in plots from which thinned fruit were completely removed than on those in plots from which thinned fruit were not removed. M. fructicola sporulated more frequently on thinned fruit placed into irrigation trenches than on those left on the dry berms in tree rows. The incidence of preharvest fruit brown rot increased exponentially as the density of thinned fruit increased on the orchard floor. These results suggest that thinned fruit left on the floor of nectarine orchards can be a significant inoculum source of secondary infections. Removal or destruction of thinned fruit should reduce brown rot in nectarine and possibly other stone fruit orchards under semi-arid California conditions.

Effects of Water Stress on Botryosphaeria Blight of Pistachio Caused by Botryosphaeria dothidea

The effects of water potential (Ψ) on spore germination, germ tube elongation, mycelial growth of Botryosphaeria dothidea, and development of Botryosphaeria blight of pistachio caused by this fungus were studied in the laboratory and greenhouse. In laboratory tests, spore germination, germ tube elongation, and mycelial growth of B. dothidea increased as Ψ decreased from 0 to -2.0 MPa and declined as Ψ decreased below -2.0 MPa. Water stress (Ψ < -0.260 MPa) increased the severity of Botryosphaeria blight on young detached pistachio leaves. In greenhouse inoculation experiments, drought-stressed 2-year-old pistachio trees (leaf Ψ < -0.635 MPa) developed more severe Botryosphaeria blight disease than the nonstressed trees (Ψ > -0.485 MPa) in both 1999 and 2000 experiments. In a similar experiment using 1-year-old potted pistachio trees in 2000, withholding water treatments did not cause enough drought stress in pistachio leaves (Ψ > -0.584 MPa) and did not result in higher disease than the nonstressed trees (Ψ = -0.466 MPa). The correlation coefficient between Ψ and disease index for 1999 experiments was 0.954, and for 2000 experiments I and II was 0.981 and 0.939, respectively. These results indicate that drought stress can be a major predisposing factor of pistachio to infection by B. dothidea. Results from this study can be used to better understand the disease outbreaks in California pistachio orchards and to develop integrated management strategies by adjusting irrigation.

Attraction of thrips (Thysanoptera: Thripidae and Aeolothripidae) to colored sticky cards in a California avocado orchard

Yellow, white, and blue sticky cards were tested in an avocado orchard for their attractiveness to Scirtothrips perseae, Frankliniella occidentalis and Franklinothrips orizabensis. Clear pieces of plastic coated with adhesive were used as controls to determine rates of random interception of thrips independent of color. Yellow was most attractive to S. perseae and white cards captured mostly F. orizabensis and F. occidentalis. Capture rates on blue cards declined across the course of four trials for S. perseae and F. occidentalis; this trend was not observed for F. orizabensis. Cardinal direction of traps did not significantly effect capture rates of thrips suggesting south westerly prevailing winds did not influence patterns of aerial dispersal. r 2002 Elsevier Science Ltd. All rights reserved.

Effects of Single-Drop Impactions and Natural and Simulated Rains on the Dispersal of Botryosphaeria dothidea Conidia

ABSTRACT Laboratory and field experiments were conducted to study the dispersal of Botryosphaeria dothidea conidia using single-drop impactions and natural and simulated precipitations. For laboratory studies, 200 single drops were released from a height of 1 m on infected pistachio nuts. On pieces of photographic film, 50% of the droplets were collected within 20 mm (average droplet travel distance) of the target area, and the droplets ranged from 0.041 to 3.19 mm in diameter, with an average of 0.3 mm. Each droplet carried an average of 23 B. dothidea conidia. In 3 years of field experiments, rainwater was collected in funnels connected to bottles positioned at different heights inside the tree canopy and at different distances away from the edge of tree canopy in three commercial pistachio orchards in San Joaquin, Yolo, and Glenn counties in California. Numbers of conidia in rainwater varied among and within sampling seasons by sampling dates and orchards. Up to 67,000 conidia/ml were obtained in rainwater samples collected from an orchard in Yolo County. Rainwater from orchards in Yolo and Glenn counties contained a consistently higher number of conidia than rainwater collected from the orchard in San Joaquin County. Variation in numbers of conidia also existed among heights where bottles were located. There were significantly more conidia in rainwater collected inside than outside tree canopies. Inside tree canopies, bottles located at 100 and 150 cm above ground collected more B. dothidea conidia than those placed at 50 and 200 cm. Conidia were collected as far as 1 m from the tree canopy edge. Based on data from the Glenn County orchard, a linear relationship between number of conidia (Y) and rainfall amount (X) in millimeters was determined as Y = 240X - 3,867, with r(2) = 0.91, which meant that a minimum of 16.1 mm of rain was needed to disperse conidia of B. dothidea. The power law model best described the dispersal gradients of B. dothidea propagules in the 1999-2000 and 2001-02 sampling seasons, with r(2) values of >/=0.73, whereas the exponential law model fit best for the 2000-01 data, with r(2) values of >/=0.81. In a rain simulation experiment, the intensity of the rain generated by a nozzle at 138 kPa of pressure inside the tree canopy was approximately five times higher than rain recorded outside the tree canopy. Rain removed up to 65% of conidia from infected fruit. These results confirmed that B. dothidea is a splash-dispersed pathogen with relatively short distances of spore dispersal within pistachio orchards. Only pycnidia are present in pistachio orchards; therefore, the results also indicate that inoculum of B. dothidea should be entirely splashed dispersed.

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.

Sensitivities of Baseline Isolates and Boscalid-Resistant Mutants of Alternaria alternata from Pistachio to Fluopyram, Penthiopyrad, and Fluxapyroxad

Resistance of Alternaria alternata to boscalid, the first succinate dehydrogenase inhibitor (SDHI) fungicide labeled on pistachio, has become a common occurrence in California pistachio orchards and affects the performance of this fungicide. In this study, we established the baseline sensitivities of A. alternata to the new SDHIs fluopyram, fluxapyroxad, and penthiopyrad and assessed their cross resistance patterns with boscalid. Examination of the effective fungicide concentration that inhibits mycelial growth to 50% relative to the control (EC50) for 50 baseline isolates revealed that the majority were sensitive to boscalid, penthiopyrad, fluopyram, and fluxapyroxad. Analysis of EC50 values for boscalid for 117 A. alternata isolates originating from boscalid-exposed orchards showed that 44, 3, 1, and 69 isolates had sensitive, reduced sensitivity, moderately resistant, and highly resistant boscalid phenotypes, respectively. Molecular investigation of the occurrence of known SDH mutations showed that, among the 69 isolates highly resistant to boscalid, 44, 2, 14, and 1 isolates possessed the mutations leading to the H277Y, H277R, H134R, and H133R amino acid substitutions in AaSDHB, AaSDHB, AaSDHC, and AaSDHD subunits, respectively. Some SDHB or SDHC mutants displayed highly sensitive, sensitive, or reduced sensitivity phenotypes toward penthiopyrad or fluxapyroxad, whereas other had low, moderate, or high levels of resistance to these fungicides. In contrast, all the SDHB mutants were sensitive to fluopyram, while 10, 5, and 1 SDHC mutants had sensitive, reduced sensitivity, and moderately resistant fluopyram phenotypes, respectively. The SDHD mutant had reduced sensitivity to fluopyram and penthiopyrad but was highly resistant to fluxapyroxad. The discrepancies of cross-resistance patterns between SDHIs suggest that their binding sites in complex II may differ slightly and that additional mechanisms of resistance to these compounds are likely involved. Ultimately, the findings of this study should lead to the rational and sustained deployment of new SDHIs in Alternaria late blight spray programs.

Antagonism of Paenibacillus lentimorbus to Botryosphaeria dothidea and biological control of panicle and shoot blight of pistachio

A potential microbial fungicide, Paenibacillus lentimorbus isolate CBCA-2, against Botryosphaeria dothidea, the pistachio panicle and shoot blight fungus, was obtained from healthy pistachio leaves by both in vitro and in vivo screening techniques. CBCA-2 caused 100% inhibition of pycnidiospore germination after 24 h incubation at 25°C. Malformation of pycnidiospores and hyphae, and lysis and swollen pycnidiospores of B. dothidea occurred in the presence of cell suspensions of CBCA-2. Among the five media tested, nutrient yeast dextrose broth significantly increased the production of antifungal compounds. Application of culture filtrates of CBCA-2 suppressed disease on detached pistachio leaves, but washed bacterial cells did not inhibit lesion development. Development of lesions on excised dormant stems was inhibited only when the culture filtrate was applied before fungal inoculation. Survival of the CBCA-2 after treatment with azoxystrobin (Abound), benomyl (Benlate), tebuconazole (Elite), propiconazole (Break), or trifloxystrobin (Flint) at the highest recommended concentration was not affected, but survival was affected by iprodione (Rovral). Spraying a suspension of CBCA-2 on pruning wounds before inoculation with pycnidiospores of B. dothidea significantly reduced infection compared with the unsprayed, inoculated controls.

Spread of Endosepsis in Calimyrna Fig Orchards

ABSTRACT Pollination of the edible fig (Ficus carica cv. Calimyrna) is mediated by a small symbiotic wasp, Blastophaga psenes, that inhabits the syconium cavity of the spring crop of fig pollinator trees (caprifigs). These fig wasps also carry propagules, mainly of Fusarium verticillioides (formerly F. moniliforme) and other Fusarium spp., which cause endosepsis, from pollinator figs to the edible Calimyrna figs in California. Spread of endosepsis was studied in one experimental and up to four commercial Calimyrna fig orchards from 1989 through 1995. The incidence of endosepsis in fruit collected from the tree canopy at either <2.0 m (low) or >2.0 m (high) height, from the north and south of the tree canopy, and from the outer (direct sunlight) and inner (shaded) canopy were similar. More wasps were captured in fig trees located 3.5 to 10 m east or west of the source than in trees 48 to 63 m from the source. In addition, significantly more wasps entered the syconia of trees closest (9 to 12.7 m) to the source than the syconia of the second or third trees (18 to 38.2 m) from the source. Endosepsis decreased with distance from the source, decreasing faster to the south than in other directions from the source. In addition, the disease-vectoring wasps decreased with increased distance from the source, which also described the disease spread from the contamination source for most directions, with a sharper decline south of the source. A 3-year study in three commercial Calimyrna orchards showed there is no secondary spread of fig endosepsis in the field. Although endosepsis can complete as many cycles (three to four) as its vector in fig pollinator trees, in Calimyrna figs it is considered a monocyclic disease. Because fig wasp pollinators prefer to stay close to the contamination source when receptive Calimyrna figs are available in close proximity, only disease sources (caprifigs trees) found among Calimyrna trees or at a distance less than 50 m from the borders of Calimyrna orchards affect endosepsis incidence in commercial orchards.

PUPATION BIOLOGY OF FRANKLINOTHRIPS ORIZABENSIS (THYSANOPTERA: AEOLOTHRIPIDAE) AND HARVESTING AND SHIPPING OF THIS PREDATOR

In the laboratory, 82-99% of late second instar Franklinothrips orizabensis Johansen larvae abandoned avocado branches and artificial branches constructed of wooden dowels and were recovered below branches trapped on tangle foot coated plastic sheets, suggesting a preference by this life stage for selection of pupation sites beneath host plants. Of three media tested (coarse and fine vermiculite, and parafilm cones) for harvesting F. orizabensis pupae in cocoons, parafilm cones were most easily harvestable from colonies, and 44% of deployed late second stage larvae that were recovered used parafilm cones for pupation in experimental cages. Harvesting and shipping trials using aspirated adult F. orizabensis or pupae in parafilm cones showed significant differences in survivorship when held in the laboratory or shipped round trip from Riverside, California to Amherst, Massachusetts. Survivorship of aspirated adults was reduced on average by 41% following shipping, and mortality was highest for adult males. Transit survivorship was increased by 53% if F. orizabensis were shipped as cocoons in parafilm cones. Inclusion of ice packs in polystyrene boxes did not significantly increase survivorship rates for F. orizabensis adults or pupae that were either retained in the laboratory or shipped. This result may have been an artifact resulting from the time of year (i.e., May and temperatures were moderately cool) when shipping trials were conducted.

Role of nitidulid beetles and vinegar flies in the sexual cycle of Mucor piriformis in tree fruit orchards

Nitidulid beetles (Carpophilus spp.) and vinegar flies (Drosophila melanogaster) from stone and pome fruit orchards readily acquired propagules of + and - mating types of Mucor piriformis from decayed fruit and transmitted them to healthy fruit on which zygospores developed. Higher numbers of decayed fruits on the orchard floor resulted in increased acquisition of + and - mating types of M. piriformis by vinegar flies and vice versa. In controlled laboratory and field experiments, nitidulid beetles and vinegar flies acquired propagules of + and - mating types of M. piriformis and carried them on healthy peach and nectarine fruits, resulting in fruit infection and development of zygospores of M. piriformis. Propagules of + and - mating types of M. piriformis persisted for at least 11 days on natural populations of vinegar flies after exposure to fruit decayed by M. piriformis. Numbers of zygospores developed on fruit exposed to contaminated nitidulid beetles and vinegar flies varied considerably from fruit to fruit as did numbers of zygospores resulting from contaminated insects plated in dishes containing acidified potato-dextrose agar. Several zygospores of M. piriformis developed on peaches as a result of insect activity germinated in 15% of the fruit placed in the field during the winter.