Yuma is in the full swing of downy season for lettuce. The light yellow, irregular leaf spots on the upper leaf surface with white to pale-gray fungal growth on the under leaf surface can be found popping up in fields everywhere. Damp and cool conditions are required for infection and symptom development. Sporangia of Bremia lactucae, the oomycete pathogen responsible for lettuce downy mildew (LDM), are wind disseminated and capable of germinating on the lettuce leaf surface they land on in 3 to 4 hours if free moisture is present. All across Yuma County we started reaching that leaf wetness threshold just about the last two weeks of November (See graphs below). Leaf wetness has remained especially high in Roll and South Yuma, while North and central Yuma have been drying out, although leaf wetness in central Yuma is on a rising trend as of the time of me writing this article.

Graphs retrieved from the Arizona Meteorological Network leaf wetness reports. Leaf wetness determined by electrical conductivity. Dry hours = number of hours less than or equal to 273 mV out of 24 hours in the day, transition hours = between 273 and 284 mV, and wet hours = number of hours greater than or equal to 284 mV.

There are 10 internationally recognized races of LDM reported in the United States. The naming of new races of LDM is overseen by the International Bremia Evaluation Board (IBEB), a global association of breeders, researchers, and regulatory bodies. Races are determined by inoculating varieties with known resistance genes to LDM and seeing which varieties the race can and cannot cause disease on. Per IBEB standards, all races have the prefix “Bl:” followed by a space and the race number. European isolates have the suffix “EU” and American isolates have the suffix “US”. For instance, Bl: 1US represents American race 1.

Bl: 1-6US, or American races 1-6, are no longer detected in the western US and are therefore not considered important targets for resistance breeding by IBEB. In spring of last year, five samples from a local favorite variety, ‘Boronda’, were found to be infected with downy mildew. ‘Boronda’ is marketed with genetic resistance to the US western races Bl: 5-9US. Two samples of ‘Boronda’ submitted by Dr. Stephanie Slinski, associate director of applied research & development at YCEDA, in February of last year from Yuma were evaluated by the UC Davis Bremia group led by Dr. Richard Michelmore. When inoculated on differentials, both isolates behaved like isolate VP-300, a non-IBEB-designated variant currently under evaluation to be designated as a new American race. VP-300 was identified in 2017 and is apparently capable of bypassing the genetic resistance bred into ‘Boronda’.

I have received reports of ‘Boronda’ resistance being broken again this year in South Yuma and the Wellton/Tacna/Roll area. ‘Muggins’ is reportedly showing susceptibility as well, and I am especially interested in collecting samples from this variety for additional race testing to see if this is also due to the presence of VP-300.

Take home messages for growers:

• Downy mildew is active and the weather is perfect for disease development around Yuma County
• New, aggressive isolates are out there and they are breaking beloved resistant variety favorites
• Genetic resistance may be less effective this year than in previous years in certain varieties, use a full IPM approach
• You can submit samples of LDM to the Yuma Plant Health Clinic for race testing, ensure you fill out the downy mildew drop off sheet completely and leave plants in the sample fridge outside the clinic

If you have any concerns regarding the health of your plants/crops please consider submitting samples to the Yuma Plant Health Clinic for diagnostic service or booking a field visit with me:

Chris Detranaltes, Ph.D.
Cooperative Extension – Yuma County
Email: cdetranaltes@arizona.edu
Cell: 602-689-7328
6425 W 8th St Yuma, Arizona 85364 – Room 109

Useful Links:
BEB website: https://worldseed.org/our-work/disease-resistance/other-initiatives/ibeb/

UC Davis Bremia database: https://bremia.ucdavis.edu/bremia_database_main.php

AZ Meteorological Network leaf wetness reports: https://viz.datascience.arizona.edu/azmet/leaf-wetness-report/