Restricted spring growth syndrome

The signs of the syndrome “The plants that are delayed in their development usually start activating their buds and generating their first shoots when the normal plants are already flowering,” says Pérez-Donoso. Initial shoot growth is compressed – shoots may have eight or nine internodes in the first 10 centimetres. Elongation normalises late in the season when the shoot eventually flowers. Affected vines have less overall shoot growth and smaller canopies than normal vines. “They never recover during the season,” notes Pérez-Donoso. “And they stay active until very late. Usually, their leaves are burnt when the first frost comes.” Only some parts of a vineyard and only some vines within that part may show signs. Different parts of the same vine may even be differentially affected. “In terms of production, this is a disaster,” says Pérez-Donoso. “Restricted growth can largely affect the formation of new clusters. Even the plants that look more normal might have yield reductions.” His group surveyed four vineyards of IFG Ten – better known by its trade name Sweet Globe – in 2022. The San Vicente vineyard has never suffered from restricted spring growth syndrome and acted as a control. The other three vineyards have all shown signs of restricted growth. All four vineyards were screened for pathogens that occur in Chile and could cause growth restriction. The researchers found no pathogenic bacteria or fungi. Viral infections were identified but didn’t correlate with the occurrence of growth restriction.

Restricted growth under the microscope Pérez-Donoso and his team compared two-year-old shoots from affected and normal-appearing vines in vineyards with restricted spring growth syndrome to shoots from the unaffected San Vicente vineyard. Shoots were cross-sectioned and stained to study the tissues under the microscope. Normal grapevine shoots thicken when cambium cells divide to form new xylem to the inside and new phloem to the outside of the shoot. Xylem mainly transports water and nutrients from the roots to stems and leaves. Phloem mainly transports photosynthetic products such as sugars. In the control vines at San Vicente, microscopic examination of shoot sections showed distinct layers of xylem and phloem interspersed with fibres. Normal cork development occurred toward the external surface of the shoots. In the growth-restricted shoots, the tissue layers were disorganised and poorly differentiated. The xylem and phloem conduits were also smaller. Pérez-Donoso thinks the reason lies in the abnormal division of cambium cells. Disruption of the vascular system will limit the water and energy supply to the developing shoot – it’s not surprising that affected shoots struggle. “The growers and technicians had the sense since the beginning that this might be related to vascular development,” says Pérez-Donoso. “They were quite right. So, I have to give recognition to the wisdom of the people who work in the vineyards every day.”

Environmental drivers Temperature data from the four survey orchards for the 2021 and 2022 seasons point to one possible cause of restricted spring growth syndrome: the unaffected San Vicente vineyard was warmer than the other three orchards. “Restricted growth correlates well with very low temperatures during the autumn and winter,” explains Pérez-Donoso. The San Vicente vineyard experienced four to seven days of sub-zero minimum temperatures, compared with more than 60 in the worst-affected vineyard. All the vineyards lost primary buds to frost, but the number of dead buds was lowest in the San Vicente vineyard – the worst-affected vineyard had more than twice as many dead buds. Pérez-Donoso also has data demonstrating the effect of water stress on growth restriction. He compared vines where irrigation was withheld after harvest to vines receiving regular irrigation. There was very little autumn rain, and pre-dawn measurements confirmed a significant reduction in soil water potential in the deficit-irrigation treatment. The research team collected shoots during winter and examined them microscopically. The water-stressed vines generated more xylem during winter than the irrigated vines. “We expected a better growth response from the deficit-irrigation treatments,” says Pérez-Donoso, “but it was totally the opposite.” In the following spring, the water-stressed vines had less growth and shorter internodes than the controls – typical signs of restricted spring growth syndrome. “We also evaluated the depth of dormancy,” says Pérez-Donoso. “The water restriction seems to result in an earlier release of dormancy, which might explain why the vascular tissue is more affected.”

A complex problem Pérez-Donoso believes that restricted spring growth syndrome is more difficult to solve than initially thought because the development of the vascular tissue is affected in more than one season. The primary mechanism appears to be a disruption of the cambium due to environmental stressors. “Several different environmental conditions are related to the problem,” he comments. “It’s a syndrome – it doesn’t have one single cause.” Vigorous growth requires functional buds on healthy vines in productive balance. Optimal spring conditions will stimulate these buds to break and grow strongly. Ideally, the vine should have a productive season during which it accumulates adequate reserves in preparation for the following spring. If grapevines have diseases or poor reserves, or if winter chill is insufficient, poor bud break and restricted growth can result. Functional buds can also grow poorly due to adverse spring conditions or pest infestations. Once a vine suffers from restricted growth, it’s at risk of a downward spiral of diminished productivity, reduced reserves, disease infections, and another season of restricted growth. Vineyards can be turned around through careful management, but vigorous vineyards can also be damaged by late-season trauma or adverse winters. “We’ve only seen restricted spring growth syndrome in young vines – two- to four-year-old vineyards,” says Pérez-Donoso. “It could perhaps happen in old vineyards, but we’ve never seen that.” This highlights the importance of not stressing young vines. Growers might not have control over the weather, but they should take care to avoid water stress, overcropping, and promoting growth too late in the season. “I think that if we start by looking at all the different stressful situations that could lead to this problem, it will be reduced,” concludes Pérez-Donoso.