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- Malady Mystery #3
Hello Robigalia Reader and welcome back to another week of Robigalia, delivering the latest in plant pathology directly to your inbox!
This week, I'm back with another instalment of Malady Mystery, where I explore diseases or conditions with unknown causes. Scroll down to dive head first into this week’s malady! If you’re a Robigalia subscriber, you also have access to my online database and today’s title song!
As a quick aside, starting next week, you will notice a change in your weekly Robigalia newsletter. Until now, I have produced an original article each week. While I truly enjoy creating this content, my PhD deadline is only three months away (gulp). As a result, I haven’t been able to devote my full attention to these articles, and I want to ensure I continue delivering high-quality content to you. With this in mind, here’s what you can expect going forward:
A weekly roundup of the latest publications, research, news, and more in the plant-pathogen space.
A monthly original article dedicated to a specific plant-pathogen interaction (similar to current editions, but with more depth).
Thank you all for your support so far; it’s a pleasure to see this community grow. Please feel free to reach out with any feedback on these changes or let me know what you would like to see in the future. Now, back to this week's malady!
Pumpkin brown etch primarily affects butternut pumpkins (Cucurbita moschata), often rendering severely affected fruit unsaleable due to its unattractive appearance. Although this condition has been troubling Queensland growers since the 1970s, its cause remains unknown. Interestingly, other varieties such as Queensland blue or kabocha (Cucurbita maxima) are generally unaffected.
Symptoms of brown etch typically begin as small, orange-brown spots on fully grown but still immature fruit. These spots almost always appear where the fruit touches the ground, a stem, or another pumpkin. The lesions can rapidly expand, sometimes covering much of the fruit's surface. As the fruit matures, the affected areas turn from brown to white, often developing concentric colour bands. Combined with surface cracking, this creates the characteristic "etched" appearance. Microscopic examination shows that cells in the etched areas have significantly thickened cell walls, primarily due to lignin accumulation. This process ultimately leads to cell death, leaving behind the whitened remnants of the cell walls.
Despite decades of observation, the precise cause of brown etch remains elusive. While there is some association with gummy stem blight (Stagonosporopsis cucurbitacearum) and possibly Fusarium species, etch can also occur in plants that appear disease-free. Water conditions seem to be the strongest predictor. High relative humidity (>90%) or consistently wet conditions due to rain or dew are major factors in the development of etch. The number of wet days leading up to harvest is often used to predict the likelihood of etch. If plants are wet for a total of 2 to 4 days in the fortnight before harvest, 5–10% of the pumpkins are likely to develop etching. If plants remain wet for 5 days or more, at least 20% of the fruit may be affected.
The incidence of brown etch can vary greatly between plantings. Field studies report disease incidences ranging from 20% to 50%, meaning the economic impact can be considerable. In cases where incidence reaches 50%, growers could potentially lose half the value of their crop, as severely affected fruit cannot be sold. With the cause still unknown, growers are limited to keeping developing fruit as dry as possible during growth and post-harvest to reduce the likelihood of etching.
When I set out to write this article, I didn’t expect such a scarcity of literature on the subject (I mainly wanted an excuse to include a GIF from The Office). Fortunately, a Master's thesis offered some valuable insights, which I’ve linked below. If you are aware of any recent literature, please drop it in the comments!
Pumpkin Brown Etch papers thesis
Firdause Alhaj Hasson’s thesis explores the transcriptomes of healthy and etched pumpkin tissue, alongside light microscopy. The research suggests that brown etch is associated with increased lignification and cell wall thickening, likely in response to increased turgor pressure in affected cells. This appears to be an active cellular response rather than sudden cell death, indicating a possible abiotic rather than biotic cause.
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Until next week,
Alyssa
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