Relative Resistance Level to Bacterial Blight in a Number of Walnut Cultivars and Selected Genotypes

The major disease of Persian walnut is walnut blight caused by the bacterium Xanthomonas arboricola pv. juglandis. This disease limits walnut production in many regions of the world in particular those with warm and humid springs. All succulent tissues including recent shoots, fruits and catkins may be attacked but their sensitivity decreases over time. Symptoms on leaves begin as small, water-soaked spot that can expand to form angular necrotic lesions surrounded by a yellow hallo. Infection on the hull of the unripe fruits is initiates as a dark spot, which rapidly creates dark sunken lesions. If fruit infection occurs before shell hardening, the kernels are usually destroyed, infection after shell hardening results in dark kernels and poor-quality nuts, while the kernel is until consumable. The main current strategy for walnut blight control is to protect succulent tissues by copper-based sprays from bud-burst. However, multiple copper sprays do not always control the disease and there is the risk of development of copper resistant bacterial strains. Cultivation of resistant cultivars is the most practical, environmental friendly and economical approach for controlling of this disease. So far, there have not been any cultivar completely resistant or immune to this disease. However, variation in susceptibility level has been shown to occur, worldwide. The objective of this study was to determine leaf and unripe fruit resistance of Iranian promising walnut genotypes in response to controlled inoculation with X. arboricola pv. juglandis in orchard and laboratory conditions.

In this research, the relative resistance of leaf and unripe fruits to bacterial blight were studied in a number of promising walnut genotypes replicated during two years of 2015 and 2016. The local genotypes have been selected through massive surveys of native orchards in different provinces of the country. The main selective criterion for walnut selection was late leafing trait. In addition, correlation between leaf blight and unripe fruit blight severities were studied. The material included H1-7, H2-1, SHK2, Vina, Round de montignac (RDM), Lara, G4, G5, B10 and K15 genotypes and cultivars. From this material, KZ3, 88-2, 88-1 and G3 were registered and released in year 2019. Also, Serr, Chandler and Hartley cvs. were used as controls. A mixture of four local bacterial isolates originated from Qazvin, Karaj, Zanjan and Urmie was used as inoculum. The bacterial strains have already been characterized using different phenotypical and molecular tests. They were cultured on Nutrient Agar (NA) plates at 28 ℃. After 72 hours of cultivation, the colonies were suspended in distilled water; the concentration of the suspensions was read by spectrophotometer and after equalization, mixed and used as inoculum. Assessing leaf resistance was performed by spraying one year old grafted seedlings in glasshouse condition and lesion area was recorded 21 days after inoculation. Three leaves (contacting leaflets) of three seedlings were used per genotype. Assessing unripe fruit resistance was achieved by inoculating 45 days old fruits in laboratory condition and lesion diameter data was read 15 days after inoculation. 15 fruits per genotype were used and three lesions were created in each fruit. Statistical analysis of data was performed by Duncan Multiple test range using SAS software.

Based on the results, leaf and unripe fruit blight severities were different among different cultivars and genotypes in both years of evaluation. Combined analysis of blight severity data indicated that G3 (Alvand) and 88-2 (Persian) genotypes had the most leaf blight severity and thus, rated as the most susceptible, while Hartley cultivar had the least leaf blight severity and thus, as the most resistant. Based on combined analysis of data, H1-1 and SHK2 genotypes had the most blight diameters of unripe fruits and thus rated as the most susceptible and resistant genotypes, respectively. Leaves of newly released cultivars of Persian, Caspian, Alvand and Chaldoran were rated as susceptible, relatively resistant, susceptible and relatively susceptible, respectively. On the other hand, unripe fruits of Persian, Caspian, Alvand and Chaldoran were classified as resistant, resistant, relatively resistant and susceptible, respectively. Mean leaf blight severities were not different between the two years of study and also there was no difference between unripe fruit severities between two years of the experiment. No significant correlation was found between leaf blight and unripe fruit blight severities.