A grape bunch contains berries and a rachis – two dissimilar tissues susceptible to different postharvest issues. A waxy coating protects berries from water loss, whereas the unprotected rachis is vulnerable to drying and browning. The rachis can also freeze below minus 1.0 °C, whereas berries freeze at minus 2.0–minus 3.0 °C. Grapes are non-climacteric fruit. Berries have a low respiration rate and, therefore, a low sensitivity to physiological disorders but a high sensitivity to senescence. Pre-optimally harvested grapes won’t ripen, but overmature grapes are more likely to suffer postharvest decay and internal browning. “Soluble solids are a measure of acceptability to the consumer – very high values could occur at a senescent stage of the tissue,” says Dr Juan Pablo Zoffoli, a postharvest physiologist in the Faculty of Agronomy and Natural Systems of the Pontifical Catholic University of Chile. “In high-acid varieties, we don’t see much decrease in acidity in the last period of maturation,” he explains. “And soluble solids are not a good indication of ripening because it’s affected by other factors, especially crop load and waiting for the berries to colour.” Acidity and total soluble solids are influenced by cultivar and growing conditions. Growers must be familiar with the values representing optimum harvest maturity for their vineyards.
Cracks in berry defences
Ripening grape berries rely on their cuticles – their waxy coating and outer skin layer – to fend off pathogenic microbes. Cultivars with denser cuticles are less susceptible to grey mould (Botrytis cinerea), especially on the berry cheeks. In contrast, thinner-skinned cultivars developed to please consumers could be easy targets for decay pathogens. “It’s very important to get this information for new varieties,” says Zoffoli, “to really understand the risks related to berry anatomy and the genetics of new varieties.” Cuticle structure is also related to microfractures. When berries expand, their surface comes under tension and can develop microfractures. The microfractures are initiated before harvest but can lead to hairline cracks appearing postharvest when berries are exposed to excessive concentrations of SO2. Affected berries are more prone to decay and can become sticky during storage due to juice exudation. Hairline cracks can be prevented using the minimum effective SO2 dose and avoiding delayed cooling and cold-chain breaks.
Risk factors for grey mould
“Most growers have been dealing with Botrytis cinerea for many years,” says Zoffoli, “but it’s still a critically important disease.” Grey mould can develop on the cheek or at the stalk end of the berry – it rarely develops at the opposite end. “When we talk about the cheek, we’re referring to the lateral infection of the berry,” clarifies Zoffoli. Lateral infections usually appear before harvest and can be removed before berries enter the postharvest phase. Rain at harvest time may exacerbate lateral infections. Applying SO2 is crucial for disinfecting the skin surface and preventing decay once the bunch lands in the humid environment of a bag. Basal infections are more problematic, especially when table grapes are stored or shipped longer. These infections will continue to develop inside berries even after SO2 treatment. Should insufficient SO2 levels be maintained, the infection will spread through the bunch. The basal infections start early on when Botrytis establishes on dead tissues such as floral debris in the developing bunch. Inappropriate use of plant growth regulators increases the risk of basal infections. “Sometimes we abuse the concentration and amount we’re using, and we delay the harvest and dilute the cuticle. In some cases, we increase berry shatter,” cautions Zoffoli. “When the stalk insertion weakens, and berry shatter increases, then grey mould infections at the base increase.” Cultivars differ in their susceptibility to different types of infections. For instance, basal infections are more common in cultivars like ‘Thompson Seedless’, in which flower debris tends to remain attached and the stalk insertion is weak. In contrast, cultivars like ‘Red Globe’ have the opposite features.
From sour rot to blue mould
Sour rot refers primarily to preharvest infections by several decay pathogens, including Aspergillus niger, Rhizopus stolonifera, and Alternaria, Cladosporium, and Penicillium species. “It’s important to understand that those infections come directly from some specific damage to the berry’s surface, especially microfractures,” says Zoffoli. Microfractures are more likely when vines are over-irrigated, or fruit are harvested at post-optimum maturity. Insect and bird damage and Botrytis infection can also predispose berries to sour rot. Once infected grapes are placed in cold storage, the primary decay pathogens are suppressed, and secondary infections with bacteria, yeasts, and blue mould (Penicillium expansum) take over. Blue mould typically manifests as a soft rot after extended cold storage. According to Zoffoli, blue mould is highly related to harvesting overmature fruit. “You need to be careful when you’re moving into the late period of the harvest window,” he warns. He collaborated on a project to assess the effect of different fungicides and SO2 on sour-rot pathogens, but none of the fungicides tested were effective for all the pathogens.
Postharvest decay control
Zoffoli summarises postharvest decay control under four headings: characterising latent infections, maintaining the cold chain, managing SO2, and using appropriate packaging. “We need to understand what is happening with latent infections,” he says. “This will tell you if your fungicide programme is working, and you can know the risk in your asymptomatic berries.” Latent infections are, per definition, not visible. They can be diagnosed using sophisticated laboratory techniques or a simple method devised by Zoffoli’s group. He suggests harvesting a bunch, sanitising it with SO2, and placing it in a bag at a low temperature so that condensation develops. The bag is then moved to a higher temperature. He has found a good correlation between grey mould at the berry base after this test and total grey mould after 45 days at 0 °C, followed by three days at 20 °C. For high-risk grapes, SO2 management is crucial. “For fumigation, we use 300 ppm per hour because we get more uniform and better control,” reports Zoffoli. “When we move to the effectiveness of the SO2-generating pad, we need to know the emission rate and keep it at the level of 1–3 ppm around the berries.” He has investigated alternatives to SO2 but concludes that none currently provide the same level of control.
Keep the rachis green
Rachis browning begins with water loss from the rachis, causing tissue damage. Tissue damage is associated with oxidation and enzymatic processes that result in pigment formation. Pigment accumulation leads to browning of the rachis. Browning can also occur due to damage from freezing. “Most of the water loss occurs in the first eight hours after harvest, so we need to control water loss during that time,” says Zoffoli. Unlike many other crops, table grapes are not precooled before packaging – water loss during the long delay before cooling is of significant concern. The interval between harvesting and cooling should be as short as possible to minimise water loss, with grapes kept out of direct sunlight after harvest. Increasing the relative humidity in the packhouse and inside packaging before cooling will reduce water loss. Rapid cooling, followed by humidity maintenance during cold storage, will further limit water loss. Zoffoli emphasises that cultivars differ in their sensitivity to rachis browning. Some can tolerate more water loss without developing browning. In sensitive cultivars, rapidly achieving low temperatures becomes more critical, even when humidity is maintained, for example, by bagging fruit. Considering the logistical challenges South African growers face, understanding how postharvest problems like decay and rachis browning develop is more relevant than ever. Table-grape producers need every tool to ensure they deliver top-quality fruit to consumers.
