Do draped nets affect plum quality?

A Hortgro-funded project assesses the effect of draped nets on external and internal fruit quality. By Anna Mouton.

There is a growing consensus among most plum technical advisers that protective nets are becoming essential for maximising Class 1 pack-outs of some cultivars. Most growers have adopted fixed or retractable overhead nets, but these are expensive and best erected when establishing a new orchard.

Draped nets are potentially a cost-effective alternative to overhead nets and are easy to implement in existing orchards, but there is not much information on their use in plums. A recent Hortgro-funded study has started to bridge the knowledge gap by examining external and internal fruit quality in two cultivars under draped nets.

The three-year project is led by Handré Viljoen, Stone-fruit Portfolio Manager at ExperiCo Agri-Research Solutions. He and his team covered the same trees in two full-bearing commercial orchards with white 20% draped nets and compared them with uncovered trees.

One trial site was a Laetitia orchard in Montagu, and the other was an African Delight orchard in Simondium. Both areas have high summer temperatures and plenty of wind. The Laetitia orchard had drip, and the African Delight orchard had micro-sprinkler irrigation.

The draped nets were applied after thinning and removed at harvest. The researchers installed loggers to measure temperature and relative humidity under and outside the nets.

They also collected data on shoot growth, yields, and fruit quality in the orchards, taking a sample of fruit to the laboratory for a complete external and internal quality assessment. Further fruit samples were evaluated after six weeks in cold storage and an additional five days of shelf life.

Data analysis for the first two seasons is complete, but is still underway for the third season.

Draped nets and microclimate

Growers commonly report more vigorous growth of trees under overhead nets, but this wasn’t observed in Viljoen’s trials. His team measured marked shoots before applying and after removing the nets in each season.

Shoot growth and yields in netted and unnetted trees were similar in the first two seasons, except for a significantly greater yield in Laetitia under draped nets in 2024. The netted Laetitia trees yielded an estimated 38.8 tonnes per hectare against 23.2 tonnes per hectare in the uncovered trees.

“The shade factor isn’t so large that you would expect big differences in fertility or vegetative growth,” comments Viljoen. However, many growers have seen increased growth and yields under overhead nets, so seeing the third season’s growth and yield data will be interesting.

The effect of the draped nets on canopy temperature and relative humidity wasn’t consistent across seasons, and neither was the weather. According to Viljoen, the first season was exceptionally warm, whereas the following seasons were more moderate. Harvest dates (Table 1) also varied substantially across the three seasons.

Daytime temperatures under nets are generally higher in Viljoen’s experience. “We’ve seen this with other trials as well, where it’s 1–3 °C warmer under fixed nets than outside,” he says.

“I think the row direction plays a role in the temperatures under the draped nets,” he speculates. When the row and wind directions are the same, the wind blowing down the rows has a greater cooling effect than when the wind only blows over the top of the orchard.

Table 1. Harvest dates during the three seasons of the draped-net trial.

External and internal fruit quality

Fruit harvested from netted trees tended to have fewer rub marks than those from unnetted trees when assessed after six weeks of cold storage and again after a further five-day shelf life.

In the first season, rub marks affected about 29% of Laetitia plums from uncovered trees, but only 14% of the Laetitia plums harvested from trees under draped nets. In the second season, the figures were 10% for uncovered trees and 6% for netted trees.

For African Delight, rub marks were found in about 27% of uncovered fruit and 9% of netted fruit after six weeks of cold storage in the first season. Neither the netted nor the unnetted fruit had any rub marks in the second season.

“When fruit is on the outside of the tree, the net rubs against them and leaves a mark,” said Viljoen. However, in his trials, the nets damaged less than 1% of Laetitia and no African Delight fruit, suggesting that wind is a far more serious problem.

“There’s definitely less sunburn under nets,” he adds. But he cautions that the nets don’t necessarily protect fruit against heat damage.

Viljoen’s overall takeaway from the first two seasons’ data is that draped nets have no adverse effects on fruit colour or quality in Laetitia and African Delight.

“Our data so far shows that draped nets do no damage,” he says. “If this season’s data is similar, then we need to do a larger-scale trial on a farm where we can put the fruit on a pack line to assess large samples and determine pack-outs.”

What does industry say about this research?

“Anything growers can do to increase their pack-outs will help to improve their income. Using nets over plums is relatively new and still being investigated and refined. At this stage, the preferred option is retractable nets, which can be opened, rather than fixed nets that remain closed, given that cross-pollination and bee activity are critical to ensure a crop in plums.

“I think draped nets might be a good alternative, and could offer greater flexibility than other nets, so they should be investigated. However, we need to assess the costs, for example, labour costs to cover and uncover the trees, as well as how frequently nets must be replaced.”

Technical adviser Karin van Rensburg. In2Stone.