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.

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.