Dynamics and Pattern of Latent Infection Caused by Botryosphaeria dothidea on Pistachio Buds

Understanding the Process of Latent Infection of Canker-Causing Pathogens in Stone Fruit and Nut Crops in California

The Botryosphaeriaceae family is considered a fungal family that includes pathogens causing latent infection of woody plants, and a number of species were identified as causal pathogens of canker and shoot blight diseases. To better understand the process of latent infection of major canker-causing pathogens in woody tissues in different tree crops important in California, shoot and bud samples were randomly collected from four tree crops: almond, dried plum, pistachio, and walnut. The previously developed DNA primers and quantitative real-time PCR (qPCR) assay systems were applied to detect six canker-causing pathogen groups, including Botryosphaeria dothidea, and species of Cytospora, Diplodia, Lasiodiplodia, Neofusicoccum, and Phomopsis. The concepts of molecular severity (MS) and latent infection index (LII) were introduced and applied to quantify the latent infection levels for these samples. Variation in incidence of latent infection among pathogen groups was observed, whereas the incidences were relatively low among species of Phomopsis and Diplodia. High incidences of Cytospora spp. were observed in two dried plum (prune) orchards. Most orchards showed high incidences of B. dothidea and Lasiodiplodia spp. and moderate incidences of Neofusicoccum spp. Variations in MS were observed among samples of the studied orchards, ranging from 4 to 8. The overall results of LII demonstrated that species of Diplodia and Phomopsis were less important in population development of canker-causing pathogens at the latent phase. Lasiodiplodia spp. were the most aggressive and had been well developed in populations among the studied tree crops. Cytospora spp. became predominant in two of the three dried plum orchards, whereas B. dothidea and Neofusicoccum spp. showed trends of increase in incidence across various tree crops. This study also demonstrated the usefulness of this sensitive qPCR approach in providing evidence of the latent phase of major canker-causing pathogens of stone fruit and nut crops at an early stage of latent infection in woody plant tissues.

Ecology and Epidemiology of Diseases of Nut Crops and Olives Caused by Botryosphaeriaceae Fungi in California and Spain

In recent decades, the cultivated area and production of nuts and olives have increased, driven by an increasing consumer interest in healthier food. Diseases of almond, pistachio, olive, and walnut crops caused by species belonging to the Botryosphaeriaceae family have caused concern worldwide. Although considerable progress has been made in elucidating the etiology of these diseases, scientific knowledge of other aspects of these diseases is more limited. In this article, we present an overview of the most important diseases caused by Botryosphaeriaceae fungi affecting almond, pistachio, olive, and walnut crops by focusing on ecology and epidemiology, primarily in California and Spain.

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.

Development of qPCR systems to quantify shoot infections by canker-causing pathogens in stone fruits and nut crops

AIMS

To develop real-time PCR assays for quantification of shoot infection levels of canker disease of stone fruits and nut crops caused by six fungal pathogen groups.

METHODS AND RESULTS

This study focused on six major canker-causing fungal pathogen groups: Phomopsis sp., Botryosphaeria dothidea, Lasiodiplodia sp., Cytospora sp., Neofusicoccum sp. and Diplodia sp., occurring in stone fruits and nut crops in California. DNA primers were designed to specifically target each of the six pathogen groups after the specificity tests using canker-causing and non-canker-causing pathogens and by using DNA sequences of other species from GenBank using blast. The quantitative real-time PCR (qPCR) systems were developed and used to quantify the infection levels of inoculated dried plum shoots.

CONCLUSIONS

For Neofusicoccum sp. and Phomopsis sp., which were used in inoculation of walnut shoots, the values of the molecular severity ranged from 5·60 to 6·94 during the 16 days of latent infection period. The qPCR assays were more efficient, accurate and precise to quantify latent infections caused by canker-causing pathogens as compared to the traditional plating methods.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrated the potential of using the developed qPCR systems for epidemiological studies on canker diseases of woody plants.

Evaluation of Two Disease Warning Systems for Botryosphaeria Panicle and Shoot Blight of California Pistachio and Efficient Control Based on Early-Season Sprays

Two empirical models to predict infection events were evaluated for control of Botryosphaeria panicle and shoot blight, caused by a Fusicoccum sp., as well as the effectiveness of early-season fungicide sprays on the control of this disease of pistachio. A model incorporating wetness duration was superior to one based solely on duration of rains ≥1 mm/h for ≥4 h and with temperature ≥11°C. The wetness duration threshold (W) for rain events ≥4 mm at a given temperature (T) for high-risk infection events was W = -7.8 + 397/T and the threshold for medium-risk events was W = -6.9 + 220/T. Wet periods interrupted by ≤12 h were added together to calculate W. In two orchards with high levels of inoculum, one high-risk event resulted in 20 to 23% blighted fruit at harvest and two or three high-risk events resulted in 31 to 80% blighted fruit. Latent infections were 0 to 1% in instances where only low-risk events (one to two events) occurred prior to collection of pistachio fruit for determination of latent infections and were 17 to 36% with one to three high-risk events. Early-season fungicide sprays in April to May effectively controlled panicle and shoot blight when applied up to 12 days before predicted infection events or 5 days after.

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.

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.

Relationships Among Propagule Numbers of Botryosphaeria dothidea, Latent Infections, and Severity of Panicle and Shoot Blight in Pistachio Orchards

Experiments were conducted between 1999 and 2001 to monitor the presence of propagules of Botryosphaeria dothidea and frequencies of latent infections on pistachio leaves and fruit clusters and to determine their relationships to panicle and shoot blight severity in commercial orchards. Numbers of B. dothidea propagules recovered from washing leaves and fruit clusters varied among the growing seasons and sampling dates. Lower numbers of B. dothidea propagules were obtained in 1999 and 2001 than in 2000. For the orchard in Glenn County, up to 75 propagules per leaf and 21 propagules per fruit cluster were recorded in 1999, compared with 365 and 248 propagules per leaf and fruit cluster, respectively, in 2000. Although more propagules were detected per leaf, the infection levels were higher on fruit clusters, suggesting that pistachio fruit is more susceptible to B. dothidea infection than leaves. Latent infections were detected as soon as leaves or fruit clusters started to expand and more infections were obtained in 2000 than 1999 or 2001. Significant (P < 0.05) relationships between propagules on leaves or frequency of infections on leaves (independent variables) and propagules on fruit clusters or frequency of infected fruit clusters (dependent variables) with r values ≥ 0.50 provide support for the role of latent infection in panicle and shoot blight later in the season. Propagules on leaves and fruit clusters were not significantly correlated to disease severity, but frequencies of latent infection on leaves and fruit clusters were positively correlated (P ≥ 0.05) with leaf and fruit disease severity under field conditions with r² ranging between 0.25 and 0.42. Quantitative relationships between latent infections and disease severity may be incorporated in a prediction model for disease development or be used to develop a risk assessment method to guide growers in their effort to control panicle and shoot blight of pistachio.