Updates on Botryosphaeria-Phomopsis diseases of walnut in San Joaquin County, California

Mohamed T. Nouri1, Florent P. Trouillas 2, and Themis J. Michailides2

1University of California Cooperative Extension San Joaquin County,

2University of California, Davis, Department of Plant Pathology, Davis

In the last several years, Botryosphaeria and Phomopsis canker and blight diseases have been observed increasingly in walnut orchards in almost all walnut-growing regions in California. Results from previous research revealed the occurrence of ten different fungal species belonging in the family of Botryosphaeriaceae and two species of Diaporthe (synonym Phomopsis) belonging in the fungal family Diaporthaceae. Fungal species in the Botryosphaeriaceae and Diaporthaceae can be found throughout the year on diseased branches and sometimes can occur together on the same branch, which is difficult to diagnose in the field. These fungi overwinter on dead branches and shoots and can develop both reproductive structures – pycnidia producing the water-splashed conidia and perithecia producing the airborne ascospores – with both spore types being dispersed during wet conditions.

Symptoms:

Main symptoms include cankers in branches and dieback of spurs resulting from infections moving from affected fruits (fruit blight) via the peduncle or shoots through leaf and peduncle scars. The infected branch turns black, cankers enlarge, and the pith of the branch is black or dark brown (Fig. 1). Growth in the pith, at least for Botryosphaeriaceae spp, moves beyond the killed woody tissues (external margin of canker) for 1 to 2 inches, which is obvious when one splits a shoot along the long axis.

Serial inoculation experiments indicated that pruning wounds are susceptible for at least four months, and the wounds of 3- to 4-year-old shoots are more likely to develop larger cankers than those of 1- to 2-year-old shoots. This long lasting susceptibility may be due to the hollow pith inside the walnut branches, which can provide a favorable condition for the fungal spores to germinate and continue to cause infection. As the infection or the pith cankers on spurs or branches continue growing during fall the dead part of the branch may become covered with a dense layer of pycnidia (Fig. 2D)

You may find dead branches in the lower canopy of orchards, which may be caused by abiotic problems that may include: shade/low sunlight or freeze. In this later case, dead branches will not show any vascular discoloration (Fig. 2). However, the surface of these branches eventually will be covered with scale and Botryosphaeriaceae fungi. 

New findings in 2020:

Despite several management practices implemented to prevent major yield and economic losses caused by Bot/Phomopsis diseases, recent field survey results showed Diaporthaceae fungi to be the most prevalent fungal pathogen isolated from diseased walnut samples in the San Joaquin County. Botryosphaeriaceae fungi were found occasionally in these orchards. Although growers are making several fungicide applications per season to control these diseases with emphasis in controlling the Botryosphaeriaceae, the persistence of Diaporthaceae spp. in walnut orchards has raised the question of whether the Phomopsis fungi have emerged as the main blight/canker/dieback disease of walnut in San Joaquin County.

A spore-trapping study was undertaken to determine when and under what environmental conditions spores of these fungi are released. Based on colony counts, the population of Botryosphaeriaceae fungi was significantly lower than that of the Diaporthaceae fungi – which corresponds to the results of the field surveys. Molecular work revealed the occurrence of three new species of Diaporthe recovered from both spore trapping and diseased tissues. The occurrence of these new species in walnut orchards represents new reports in California. In this spore-trapping study, we analyzed the correlation between precipitation events, irrigation, and grinding of infected branches between tree rows (following maintenance/cleaning pruning) and Bot/Phomopsis spore release. Among these variables, we found a strong correlation between spore release and precipitation: as precipitation increased, spore release also increased, and spores were mainly captured from March to May, a period that coincided with late season rainfalls.

We also detected high aerial dissemination of Phomopsis spores after grinding of the prunings (among which were also infected branches) which were placed between tree rows in a mature orchard with a high incidence of Phomopsis disease (Fig. 3); this information is of great importance as it helps to identify production practices responsible for the spread of these fungal pathogens within walnut orchards.

In orchards with sprinkler irrigation systems, low number of spores were captured during and following the first irrigation of the season. In addition, our results showed no correlation between further in-season irrigation events and the release of fungal spores of Diaporthaceae and Botryosphaeriaceae fungi. However, the wetness/humidity in the orchard resulting from the first irrigation may cause spores to ooze and be released from pycnidia in diseased tissues within the orchard.

Ongoing research:

Based on the spore-trapping study, a new fungicide program was initiated this year to investigate whether an early spray timing would be effective to reduce the disease incidence. Pruning wound protection trials were also initiated this year to evaluate the efficacy of some old and new chemical and biological compounds to protect pruning wounds from infections by canker pathogens.

Disease management practices:

Cultural control:

·        When pruning dead branches, pruning cuts should be made into healthy green wood during the summer or immediately following harvest allowing enough time before rains occur and spread inoculum to susceptible fresh cuts.

·        For young orchards not infected with Bot/Phomopsis pathogens, after pruning (pruning for training), you can shred prunings and leave wood chips in orchard. No sprays are needed.

·        For heavily infected orchards, it is advisable to remove out infected prunings from the orchard and shred or burn them if permitted.

·        For orchards/trees affected by the November 2020 freeze damage, remove dead limbs and prunings from the orchard as they may eventually be infected with Bot/Phomopsis pathogens.

Chemical control:

·        Timely application of effective fungicides adjusted for weather and Bot/Phomopsis inoculum level in orchard.

·        In orchards with a high incidence of Phomopsis, emphasis should be given to include a triazole fungicide in the spray program.

·        It may be good to consider applying a Bot/Phomopsis spray before the first irrigation of the season. Irrigation may create a microclimate that encourages potential infection and sporulation of these fungal pathogens

Fig. 1. Symptoms in walnut trees associated with Botryosphaeria and Phomopsis fungi; (D, shows the growth of the fungi within the pith beyond the margin of the canker - dead tissues).

Fig. 2. Symptoms in walnut trees associated with abiotic problems that may include: shade/low sunlight or freeze

Fig. 3. High aerial dissemination of Phomopsis spores when grinding of infected branches that are placed between tree rows in a mature walnut orchard.