Pathogen Profile #12

Hello Robigalia Reader and welcome back to another week of Robigalia, delivering the latest in plant pathology directly to your inboxes.

While it may be an odd source of inspiration, the idea for this week’s article came from watching The Silence of the Lambs. If you recognise this famous scene, you know exactly what plant we are discussing today! Scroll down to read this week’s article. If you’re a Robigalia subscriber you can also access my online database and today’s title song!

Chocolate spot disease is one of the most destructive diseases of the widely planted faba bean, Vicia faba. Chocolate spot is caused primarily by Botrytis fabae, and less frequently by Botrytis cinerea. A recent Disease Note also confirmed Botrytis eucalypti as a causative agent in China. The disease has been reported in all faba bean cultivation areas including Africa, Asia, Australia, the Americas, Europe, and the Middle East. Although first described in 1929, the origins of the pathogen are still unknown. With the domestication of faba bean during the late 10th millennium BC in northwest Syria, the disease is believed to have originated in this region.

Botrytis fabae spreads rapidly in mild humid conditions, initially introduced to a crop via infected seed or survival in the soil as sclerotia (hardened fungal mycelium). Within 4 - 5 days following initial infection, B. fabae produces new spores on infected plant tissues. These asexual conidia spores spread easily via wind or rain, initiating new infections. The resulting disease manifests in two stages. During the non-aggressive phase, small, discrete reddish-brown spots appear on leaves and stems, giving the impression of a chocolate dusting. In the aggressive phase, spots darken and coalesce, forming larger grey-brown lesions that cover entire plant parts.

Faba beans are a crucial source of protein and nutrients in many regions, particularly in the Middle East and parts of Africa. Significant yield losses due to chocolate spot disease can threaten food security by reducing the availability of this important crop. The disease's impact on yield and quality can lead to higher prices, making it more difficult for low-income families to afford this nutritious food source. In Ethiopia, yield losses have exceeded 65% while in regions of Australia, reductions of up to 50% have been observed. Severe infections can lead to complete crop failure, further exacerbating economic losses for farmers. The disease also affects the quality of the harvested beans. Infected seeds often exhibit reddish-brown discolouration, lowering their market value and making them less desirable for consumption and planting.

Controlling chocolate spot disease relies on an integrated management approach. At the forefront is the development of resistant faba bean varieties, alongside ongoing research into the genes responsible for resistance to B. fabae. Cultural practices also play an essential role. This includes sowing clean seeds, crop rotations, and intercropping, all to reduce inoculum buildup over time.

If you’re interested in diving into the science about B. fabae, I’ve summarised a few articles below. Robigalia subscribers can scroll down to access my online database, which includes full summaries of these and all other featured Robigalia articles.

Botrytis fabae papers

Identification of Faba bean genetic loci associated with quantitative resistance to the fungus Botrytis fabae, causal agent of chocolate spot

• Researchers have identified multiple genetic loci associated with partial resistance to chocolate spot disease in faba beans, offering new avenues for breeding more resilient varieties. By mapping quantitative trait loci (QTL) and analysing gene expression, the study highlights the potential for integrating genetic markers into breeding programs to enhance disease resistance and reduce reliance on fungicides.

Management of chocolate spot of Faba bean (Botrytis fabae) through fungicides and host resistance in North Shewa, Central Highland of Ethiopia

• New research from Ethiopia offers promising insights for combating chocolate spot disease in faba beans. By integrating resistant varieties like Gebelicho with targeted fungicide applications, particularly Copper Oxychloride, researchers achieved significant reductions in disease severity and substantial yield increases. This study highlights the power of combining host resistance and chemical control to enhance faba bean productivity, providing valuable strategies for producers.

Genome Comparisons between Botrytis fabae and the Closely Related Gray Mold Fungus Botrytis cinerea Reveal Possible Explanations for Their Contrasting Host Ranges

• Researchers have sequenced the genome of Botrytis fabae and compared it to its broad-host-range relative Botrytis cinerea. The study reveals B. fabae has a reduced gene count and fewer secondary metabolite biosynthesis clusters, likely due to gene losses and degeneration, which may explain its narrow host range. These insights could inform breeding strategies for disease-resistant faba beans and improve our understanding of pathogen evolution.

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Until next week,

Alyssa