Epidemiological approaches to the control of walnut blight disease

Buds of untreated trees, untreated trees exposed to artificial rainfall, and trees exposed to artificial rainfall and also treated with Kocide + Manex with Breakthru at bud break and again with Kocide + Manex alone one week later were dissected starting about 2 weeks prior to bud break and continuing until shoot expansion to determine the process by which inoculum of X. c. juglandis, which is primarily found within buds, moves to developing shoots and nuts. The highest numbers of cells of the pathogen were found in the outermost bud scales, cataphylls, as well as the outmost immature leaves within buds. Those innermost undeveloped leaves within walnut buds and the meristematic site were usually free of pathogen until buds opened and had very low populations for several days after bud opening. Unlike in previous years where the incidence of contamination of newly emerging leaves only gradually increased with time after bud break on untreated trees to only about 50% incidence of colonization, in 2005, colonization increased progressively from about 35% of the leaves colonized at bud break to most leaves being colonized by mid-April. This process of contamination was delayed on buds from trees treated at the time of bud break with Kocide + Manex. While in 2004 the incidence of leaves contaminated with X .c. juglandis increased with time, the population size on those leaves that became contaminated did not increase appreciably even though leaves were wetted repeatedly with artificial rain, in 2005 relatively large population sizes of X .c. juglandis developed by late April (> 10 cells/leaf). This suggests that while infestations of the outermost leaves within a bud supplies inoculum for epidemics of walnut blight, this inoculum must move to the developing shoot tip and nuts at the time of (or after) bud break, providing evidence as to why the disease is somewhat dependent on moisture. More frequent rainfall events in 2005 may be responsible for the enhanced colonization seen in this year compared to all previous years of observation. In another study, thousands of buds were tagged as they opened and we applied a single bactericide spray at various times relative to bud opening. Since we knew the stage of bud/shoot development relative to the time of spray we could follow pathogen populations on buds of different phonological states. Pathogen populations were much more reduced in April on leaves and nuts developing from buds that were sprayed immediately after they opened compared to buds that opened either before of after spraying. Inoculum of X.c. juglandis can rapidly move onto developing leaves and nuts after bud break and that protective bactericide sprays are best applied at this critical time. Leaf scales and cataphylls, unlike leaves, were observed to imbibe large quantities of suspensions of Kocide + Manex when applied in 0.5% Breakthru, suggesting that the effectiveness of early-season sprays may be due at least partly to the ease with which the pathogen can be exposed to the bactericides in such tissues. Large increases in population sizes of X.c. juglandis were observed on both treated and untreated leaves and nuts in May, associated with frequent rainfall events; while populations of the pathogen were generally lower on treated trees than on untreated trees, such increases in pathogen abundance throughout the growing season had never been seen before and was probably related to the exceptionally frequent rainfall events in late April and in May. Those buds that opened earliest in the Chandler orchard were far more likely to produce a nut than those buds that opened even a week later. This suggests that we might achieve best control of nut infections by timing early season sprays based on the earliest buds that open in an orchard. The proportion of nuts that were evaluated on trees in early June that exhibited walnut blight disease was very high (>60%) on untreated control trees, and single applications of Kocide + Manex provided little apparent disease control. However, since equal numbers of buds had been tagged on treated and untreated trees we could account for nuts that had apparently dropped from the trees after infection but before rating; trees treated once with Kocide + Manex near bud break harbored about 40% more nuts than did untreated control trees. Thus early-season bactericide treatments apparently provided much more disease control that was estimated from the incidence of infected nuts due to the high numbers of infected nuts that apparently dropped from trees before rating.