With the wet weather that has already marked this growing season and more rain in the forecast for parts of California, we have been keeping a close eye on disease symptoms. While we do not yet have definite conclusions to all of the symptomology we've observed, we believe it may be of interest and utility to share some of the disease notes and photos we've collected to date. We are grateful to the new UCCE Specialist in Plant Pathology, Dr. Cassandra Swett, who has been helping us to make sense of some of what we're been seeing in the field. Included below are some general notes followed by some more specific notes from Agronomy Advisors Nicholas Clark (San Joaquin Valley) and Konrad Mathesius (Sacramento Valley).
In last week's newsletter from the California Wheat Commission, Dr. Jorge Dubcovsky, UC wheat breeder, provided an update about a new stripe rust race (MEX16.04) exhibiting virulence at CIMMYT in durum wheat on the Yr24/Yr26 genes, which had previously conferred resistance. Among CA-cultivars present in the CIMMYT screening nursery, Kronos, Kofa, Westbred 881 and Desert King have increased susceptibility to this new race (100%, 100%, 90% and 20% infection, respectively). Desert King-HP, which has the YR36 gene, had 10% infection and Miwok did not appear to be susceptible. This race has not been observed in California, so we encourage you to monitor your durum fields and let us know about any increase in infection compared to previous years.
Within our variety trials at Davis, varieties with known stripe rust susceptibility have recently started to display early stages of infection. An update this week from Dr. Xianming Chen at Washington State University indicated that stripe rust is occurring widely in the Pacific Northwest.
Multiple agents can cause leaf spotting in small grains including fungal disease such as tan spot, Septoria/Stagonospora nodorum blotch (SNB) and Septoria tritici blotch (STB). In addition, chloride deficiency can result in symptoms that are sometimes referred to as physiological leaf spot, and there are several references to chloride deficiency symptoms being more pronounced in durum and winter wheat than in spring wheat. We are seeing some of these symptoms at our variety trial in Davis and with greater prevalence in our durum trial than our common trial, and are following up with tissue and soil analysis. As noted below, we have confirmed the presence of Septoria tritici in some other fields, but no confirmation of such at this field as yet.
About 3-4 weeks ago there was a surge of aphid pressure at a number of our variety trial sites, and we are now seeing barley yellow dwarf symptoms appear as well as twisted flag leaves and grain heads associated with damage from Russian wheat aphid.
San Joaquin Valley Notes, Agronomy Advisor Nicholas Clark
Leaf Spot Symptoms Observed in North Kings County Durum Wheat:
I had a call from a PCA in Hanford about leaf spot type symptoms in Durum wheat (see Figures 3-5). There are at least two apparently distinguishable lesions: one begins as a chlorotic fleck that develops a tan or brown spot center which expands to a tan/whitish, irregular blotch with a brown ring, always maintaining a chlorotic halo (Figure 4, black arrow). Then, there is one which appears to begin as an ash/white fleck which develops a dark brown ring with little to no noticeable associated chlorosis (Figure 4, red arrow). The variety pictured here is Volante, which has documented resistance to Septoria tritici Blotch (STB) of Wheat – Mycosphaerella graminicola/Septoria tritici. There has not been a fungal pathogen species identification performed on these samples, but the symptoms also resemble Tan Spot of Cereals – Pyrenophora tritici-repentis – and/or plus Stagonospora nodorum Blotch – Parastagonospora nodorum.
The whole field is unevenly affected. Some areas (probably up to 10% of the field) are very severely affected with symptoms all the way to the flag leaf. On these plants, more mature lesions with spore development can be observed in the lower canopy. It is likely that there was an early infection with continuous secondary infection that's progressed to the flag leaves. The crop is in the boot/early heading stage. There has been a lot of 10-20 mph wind in the region recently, and there is rain in the short and long forecast. Fungicide treatment was highly suggested for this field to protect the yield potential that still existed. There are many active ingredients that have activity against the leaf spot complex of diseases in small grains, so working with a PCA to identify a registered product is a good next step for this field. An application ahead of the rain anticipated in the coming weekend, when the fungus is expected to be active, will likely be most effective. Additionally, it was suggested that 30-60 lbs N as urea/acre be applied simultaneously, depending on the grower's nitrogen program, to ensure adequate soil N supply to the plant to improve grain protein percent at harvest.
Sacramento Valley Notes, Agronomy Advisor Konrad Mathesius
1) Delta area: Wheat, septoria and waterlogging. Correlation with low-lying areas. Grower suspected bromoxynil due to rapid drop in temperature after application. UCCE Specialist Brad Hanson didn't think symptoms looked like bromoxynil. Plant pathology confirmed presence of septoria.
2) Glenn County: Wheat, waterlogging and weed pressure from last years' volunteers leading to stunting in growth and a poor stand. Volunteers were sprayed with Simplicity but timing may have been off or quantities not enough.
3) Dunnigan area: Feed Barley, waterlogging in clay soils that greened up after things got drier.
4) Malting barley (same field different soils, loamy texture), spots with necrosis in the centers on leaves. Currently being cultured by plant pathology (No photo).
5) Dunnigan area: Powdery mildew in malting barley (Copeland). Confirmed by plant pathology department (No photo).
6) Colusa wheat variety trial: Some of the same ‘tan spot' symptoms as those in the UC Davis trial affecting multiple varieties.