Preventing plant and tree diseases

Globalization and climate change are increasing our risk in the Netherlands of plant diseases that can sometimes have far-reaching consequences. In fact, the bacterium Xylella poses such a serious threat that the sector, together with the Ministry of Agriculture, Nature and Food Quality and several municipalities, simulated an outbreak in 2022. In fact, an actual outbreak of this bacterium could have major economic and financial consequences because of the far-reaching measures that would then have to be taken. We asked Jaap Smit for advice. He is a tree consultant with Plantkundig, curator of Trompenburg Gardens & Arboretum and a member of the Dutch Dendrological Society (NDV), where he was also a board member for 15 years.

The Xylella bacterium causes blockage of vessels, which can cause parts of plants to dry up. In a severe infestation, the plant can die. "There is too little awareness of the major risks posed by Xylella," he said. The European Union outlined measures in August 2020. In the event of a Xylella outbreak, all Xylella susceptible plants within a radius of at least 50 meters around the source of infection must be destroyed. A buffer zone of 2.5 kilometers is also established. Plant transport bans may apply for a short or longer period of time. This is particularly galling for growers and garden centers. The bacterium has more than 350 host plants, including common species such as lavender, periwinkle, grape and oak. The problem is that Xylella is difficult for laymen to diagnose with certainty. It can take more than a year for symptoms to become apparent.

The bacterium Xylella is present in the heel of Italy in Corsica and the Spanish Islands in the Mediterranean Sea. Between the growing area where many plants are grown in Italy (Pistoia) and the area in southern Italy is more than seven hundred kilometers. "But there is trade between the regions and contamination from the south is not inconceivable. It is therefore incomprehensible to me that there are so many plants in garden centers that have just been imported, albeit from more northern regions, from Italy," Smit sighs.

This brings Smit to the issue of awareness and responsibility. "There are still green professionals who are unaware of the impact of any outbreak. And awareness among citizens should also grow. Imagine if someone brought infected cuttings of oleander or acacia from their vacation address in Italy! To minimize risks of Xylella, I make sure that plants applied through me have been grown no more than 250 kilometers from their future location."

Another nasty disease is ash branch mortality. It is caused by a fungus that probably came with Fraxinus mandshurica from the Himalayas. Control is unfortunately not possible. Meanwhile, another danger from Asia lurks, namely the ash weevil. "This beetle was probably imported with packing wood of ornamental stones and has already killed hundreds of millions of ash trees in the United States. The beetle has also struck hard in Russia and Belarus, and I understand it has now reached the Polish border. The question is not if but when it will arrive in the Netherlands. So don't plant any ash trees for a while, except Fraxinus mandshurica, because if healthy, it is resistant to ash borer mortality and the ash weevil."

"To spread risk, variety is important. Go for as many different species as possible in your planting plan. And because of climate change, definitely consider species from southern or eastern land climates as well. So plant knowledge is actually the solution, which is why I advise green professionals to join associations like the NDV."

An annual NDV membership is €66 for companies and institutions.
Learn more: www.dendrologie.nl.