VARIETAL SUSCEPTIBILITY OF ENGLISH WALNUTS TO WALNUT ANTHRACNOSE DISEASE (Ophiognomonia leptospyla)

William W. Coates

ABSTRACT Walnut anthracnose disease, caused by the fungus Ophiognomonia leptostyla, severely impacted Central Coast walnut orchards in 2011 and 2012 with substantial defoliation of trees. Walnut anthracnose disease susceptibility was surveyed in commercial orchards in San Benito County in 2011 and 2012. Fewer observations were available in 2012 due to many growers applying fungicides for walnut anthracnose disease control. ‘Serr’, ‘Payne’ and Northern California black walnut rootstock were found to be highly susceptible. ‘Hartley’, ‘Howard’, ‘Chandler’ and ‘Bonturi’ (64-57) were found to be moderately susceptible. ‘Tulare’ appeared to be slightly susceptible. OBJECTIVES Walnut anthracnose disease (synonym = walnut leaf blotch) was first observed in the Central Coast district infecting English walnut leaves in 2010. It is caused by the fungus Ophiognomonia leptostyla (Fr) (previously Gnomonia leptostyla, imperfect stage Marssoniella or Marssonina juglandis). In 2011, the disease reached epidemic proportions infesting most orchards. In 2012, the extent of damage was more variable (perhaps due to applications of fungicides for walnut anthracnose control) but still significant in many orchards. Varietal susceptibility of English walnut to walnut anthracnose disease in California has not been studied, so a survey of foliar damage in English walnut varieties was conducted in 2011 and 2012 in several Central Coast orchards.

SIGNIFICANT FINDINGS

1. The walnut varieties ‘Serr’, ‘Payne’ and Northern California black walnut appear to be highly susceptible to walnut anthracnose disease.

2. The walnut varieties ‘Hartley’, ‘Howard’, ‘Chandler’ and ‘Bonturi’ (64-57) appear to be moderately susceptible to walnut anthracnose disease.

3. The walnut variety ‘Tulare’ appears to be slightly susceptible to walnut anthracnose disease.

PROCEDURES Orchards were surveyed for walnut anthracnose foliar damage in August and September of 2011 and 2012. Many observations were made in 2011 but in 2012 the number of observations was reduced mainly because many growers sprayed fungicides for walnut anthracnose disease control and observations were not made in those orchards. Each variety was rated on a scale of one to five with 1= no disease, 3 = moderate foliar damage with some defoliation and 5 = severe disease with significant defoliation and nut damage.

California Walnut Board 389 Walnut Research Reports 2012 RESULTS The results of the survey are shown in Table 1. Results were generally consistent across both years.

DISCUSSION

Walnut anthracnose disease has become the most significant disease of English walnuts in the Central Coast district of California during the past two years. There are obvious differences in varietal susceptibility to walnut anthracnose disease among English walnut varieties. Among the common varieties grown in California, ‘Serr’, ‘Payne’ and Northern California black walnut rootstock were the most severely affected with many leaves infected and substantial defoliation. Some young Northern California black walnut seedlings died following defoliation. ‘Hartley’, ‘Howard’, ‘Chandler’ and ‘Bonturi’ (64-57) are somewhat susceptible with moderate leaf infections and some defoliation. ‘Tulare’ appears to be slightly susceptible to walnut anthracnose infection. The blackline resistant (CLRV hypersensitive) numbered selection ’92-016-1’ was highly susceptible to walnut anthracnose. Blackline resistant varieties should be screened for walnut anthracnose susceptibility due to their Northern California black walnut heritage. More observations of Paradox rootstock need to be made before its susceptibility can be categorized. Anecdotal reports from growers (not observed by the author) indicate that ‘Ashley’ variety may also be highly susceptible.

Walnut anthracnose appears to be favored by rainfall during the dormant period and growing season. The winter and spring of 2010-11 was moderately wet. The winter and spring of 2011-12 was very dry but substantial infections still occurred perhaps due to high population of spores from overwintering, infected leaves. The summer of 2012 in the Central Coast was moderately cool with many mornings of low overcast and heavy dew that may have contributed to summer infections.

Additional varietal foliar disease observations need to be made in future years to confirm these observations. Nut infestation susceptibility should also be documented. The impact of defoliation and direct nut damage should also be investigated as to how it impacts yield and nut quality. French research has suggested leaf removal or destruction during the dormant season may help reduce infections and this should be investigated in California. Current research is addressing the efficacy of conventional fungicides – this should also be pursued for organically- acceptable alternatives.

California Walnut Board 390 Walnut Research Reports 2012 Table 1: Walnut Anthracnose Disease - Foliar Disease Ratings (San Benito County)

Variety Rating 2011 # of Observations Rating 2012 # of Observations

Serr 4.5 4 4.0 3 NC Black NR NR 4.5 2 92-016-1 4.2 5 NR NR Payne 4.0 1 4.1 2 Paradox NR NR 3.5 1 Hartley 2.7 3 2.7 3 Chandler 2.4 9 2.2 2 93-045-1 2.3 3 NR NR Pedro 2.0 1 NR NR Howard 2.0 4 2.5 1 Bonturi (64-57) 2.0 3 NR NR Tulare 1.3 3 1.5 2

1= no disease 3= moderate foliar symptoms with some defoliation 5=severe foliar and nut symptoms with substantial defoliation NR = not rated

California Walnut Board 391 Walnut Research Reports 2012