Cooperative research on walnut blight between UC Riverside and the UC Davis breeding program has led to several ways to evaluate new genotypes for blight resistance. The standard way is using natural incidence of walnut blight on trees that are old enough to have developed enough fruit that can be evaluated. This takes four to five years. Another method is to inoculate fruit, allowing susceptibility data to be collected regardless of rainfall events. Still, we have to use fruit for this assay and thus, the timeline for obtaining information on host susceptibility is not shortened. The third approach we are using for evaluating new genotypes for blight resistance is to inoculate buds at the beginning of the season and determine if the buds can support a pathogen population until the end of the season. Results show that many of the genotypes that support a high bud population also have a high incidence of fruit infections. Still, there are a few genotypes that do not support a bud population, and yet have high disease. These genotypes may have other ways for the pathogen to survive, such as cankers. Genotypes with traits that both restrict survival of the pathogen in their buds and have low fruit disease can be further advanced in a breeding program.
At the beginning of the growing season as buds and shoots grow, the epiphytic pathogen can be carried out from between the bud scales onto the new growth. Catkin and pistillate flower infections can arise from primary inoculum in healthy buds or cankers. Fruit infections that arise from the stylar end are classified as “end blight” due to the most common symptom being sunken black lesions at the flower end of the nut (photo 4). This type of infection is characteristic of a primary infection resulting from infection at the stylar end of the female flower. If wet, rainy, conducive conditions for disease exist, twig cankers can also be primary inoculum sources for catkin and pistillate flower, as well as fruit infections. Newly blighted tissues (male and female flowers, fruit with “end blight”) can serve as secondary sources of inoculum. “Side blight” is a typical symptom of secondary infections (photo 5). End blight infections will typically kill the developing kernel resulting in a dropped nut, whereas secondary side blight infections, if they occur later in the season or after the nut is fully formed, do not typically result in a dropped walnut but may predispose the nut to worm damage or result in off-graded kernels.
Kasugamycin is labeled as Kasumin for managing walnut blight and some of the label restrictions and guidelines are shown in Table 2. Applications should be initiated when conditions favor disease development. This is the same timing as for copper-mancozeb. In orchards with a history of the disease and when high rainfall is forecasted, applications should be initiated at 20-40tkin expansion. Under moderate/low disease pressure (i.e., low rainfall forecasts and minimal dews), applications should start at 20% prayer stage (leaflets unfolding, before expansion), and at 40 % prayer stage when disease pressure is very low. These stages correspond to pistillate flower emergence.
Having copper-mancozeb last in the rotation will also provide the longest lasting residuals of both active ingredients. Furthermore, with new growth increasing the canopy volume weekly in the spring as walnut trees come out of dormancy, multiple and frequent applications are necessary for most cultivars flowering and fruiting in potentially high rainfall periods.
Walnuts grow best in deep, well drained, fertile soils. Because walnut has deep roots, orchard locations should have at least 5-6 feet of permeable soil. Shallow soils, or areas with high water tables, should be avoided when planting an orchard. The English walnut thrives in a moderate, temperate climate and has limited tolerance to low temperatures in winter. Early fall frosts or severe winter freezing can kill entire branches and result in substantial damage. The structure of young trees can be severely damaged if the scaffolds are killed by cold temperatures. Late spring frosts can reduce fruit set by damaging flowers or young nuts.
In California, walnuts flower in late spring, from April through early May. Walnut is a deciduous monoecious tree with separate male (catkin) and female (pistillate) flowers. Pollen is wind dispersed. Pollen dispersal and pistillate flower receptivity occur at different times on the same tree, and cultivars vary substantially in mean flowering times. As a result, nut set is improved by planting at least two cultivars with overlapping pollen dispersal and female flower receptivity. Nuts mature throughout the summer and are harvested in the fall from mid-September through early November.
Diet samples, feces, and urine were analyzed in duplicate for energy by adiabatic bomb calorimetry (Parr Instrument Company). Nitrogen in samples was measured by combustion (CN 2000, LECO Corporation). Protein was calculated from nitrogen content (6.25 g protein/g nitrogen for base diet, urine, and feces; 5.3 g protein/g nitrogen for walnuts). Petroleum ether extraction was used to analyze dietary fat and fecal fat (Foss). Ash was determined in a muffle furnace. Total carbohydrate was determined by difference. Serum lipids and lipoproteins (total cholesterol, LDL cholesterol, HDL cholesterol, and TGs) and plasma glucose were analyzed on the Vitros 5,1 FS chemistry system (Ortho-Clinical Diagnostics).