What was it all about?

From 2016 to 2019, this investment tested a range of products and techniques to reduce the occurrence of pumpkin brown etch, also known as ‘rust mark’ in pumpkin crops.

Australian butternut pumpkin growers are familiar with brown etch, or rust mark. Although eating quality is unaffected by the condition (the brown areas are purely superficial), the appearance of etch greatly reduces crop value. In some cases, it may not even be worth harvesting.

Brown etch develops in the field, usually from a contact point with the soil, a stem, or other pumpkins. Etch appears as either a pattern of concentric brown rings, or as irregularly shaped brown blotches spreading across the fruit. Symptoms can also develop after harvest, so that a freshly packed, clean bin of pumpkins at the farm can be riddled with etch by the time it arrives at the wholesale markets.

Causes

The cause of etch is not clearly known, although it is most likely to be triggered by stress. Etch can be associated with infection by gummy stem blight or ‘black rot’ (Stagonosporopsis cucurbitacearum). However, etch also occurs when plants appear totally free of this disease, with no other symptoms of infection or evidence of fungi from RNA analysis or microscopic examination.

There is strong correlation between etch and wet weather, with wet conditions due to rain or dew strong predictors of etch risk in a crop. The project team found that the best way to reduce development of etch is to keep relative humidity low and the crop as dry as possible. This could mean increasing plant spacing, avoiding planting in damp areas, or growing with subsurface drip instead of overhead irrigation.

Transport and storage

If there is little or no etch in the field, symptoms are extremely unlikely to develop after harvest. However, if etch is observed in the field, then more is likely to develop during transport and storage. New blemishes can appear overnight on previously clean fruit, as well as continuing to expand on already affected fruit.

It may be best to store harvested pumpkins for a week or two before re-packing into hat bins or crates. By this time development of new etch will be minimal, allowing effective grading of the remaining fruit. Development of etch during transport and storage may be reduced by keeping relative humidity very low and, potentially, by cooling fruit.

Retail

A small retail study looked at consumer preferences. Most butternut pumpkins are sold cut in half and overwrapped, with undamaged flesh clearly displayed, despite the brown stain on the skin. When etched fruit was discounted by 50 cents per kilo, 12% more etched fruit was sold than clean pumpkins. Even without a discount, etched pumpkins still sold well. This suggests that if people can see the flesh is good to eat, purchasing will be minimally affected.

ACT NOW

Read this fact sheet Understanding brown etch of pumpkins produced by the project team, which discusses the causes and how best to reduce occurrence.

These articles also appeared in various industry publications, sharing information about the project and its outcomes:

• Understanding brown etch of pumpkins, published in Vegetables Australia magazine, on page 38 of the Autumn 2020 edition.
• Project update: Managing brown etch in pumpkins, published on page 36 of Vegetables Australia magazine, May-June 2018.
• Attention pumpkin growers: Have you seen this etch?, published on page 18 of Vegetables Australia magazine, March-April 2017
• Have you seen this etch?, published on page 22 of WA Growers magazine, Summer 2016 edition.

The project team is always keen to hear from growers who have seen or who are currently experiencing brown etch in their crops. Contact Dr Jenny Ekman at AHR on 0407 384 285 or at jenny.ekman@ahr.com.au.