Malady Mystery #1

Hello Robigalia Reader and welcome back to another week of Robigalia, delivering the latest in plant pathology directly to your inboxes.

For the last few months, I’ve introduced you to a new pathogen and host every week. This week, we're taking a different approach by exploring a mysterious disease without a known cause. Scroll down to read all about a mystery disease impacting 90% of some mango orchards in Australia.

Since mid-2017, mango growers in the Northern Territory, particularly around Darwin, have been grappling with a mysterious disease known as mango twig-tip dieback (MTTD). This disease has been observed in 15 orchards within the Litchfield area and is suspected to be present in many more. Unlike the common branch/twig dieback, MTTD specifically affects the tips of mango twigs, causing significant damage and impacting mango production.

MTTD has had a devastating impact on mango production, particularly affecting 90% of Darwin’s Kensington Pride orchards. The disease has led to a significant reduction in yield, with some trees failing to produce any fruit. In severe cases, entire orchards have been bulldozed due to the ineffectiveness of treatments. MTTD symptoms include black lesions running lengthwise on the youngest twig flush, typically on one side of the twig. These lesions progress both upwards and downwards, eventually killing the apical bud. Internally, the lesion can extend a few centimeters beyond the visible symptoms, causing the twig to die.

Efforts to identify the causative agent of MTTD have to date been unsuccessful. Research carried out by the Northern Territory Government isolated 150 fungal and bacterial cultures from over 20 infected mango samples. However, pathogen inoculation trials with several putative causative agents failed to result in disease symptoms in infected plants. Further testing using next generation sequencing identified thousands of bacterial and fungal species from infected plants. These results identified putative caustivie agents, inlcuding two Pantoea bacterial candidates, and Colletotrichum gloeosporioides and Cladosporium cladosporioides as fungal candidates. As the causative agent of MTTD remains unidentified, disease control has proven challenging. Despite spraying fungicides, cutting limbs off infected trees, and boosting tree nutrition, producers have failed to limit the spread of the disease.

While MTTD has only been officially identified in the Darwin region since 2017, it has quickly become widespread in that area. This rapid progression indicates that the disease could potentially spread quickly in other mango-growing regions if introduced. As transmission mechanisms also remain unknown, the disease raises concerns for major mango producing regions, especially Australia’s close neighbours in South East Asia. Ongoing research and vigilant monitoring in mango-growing areas worldwide will be crucial in managing this potential threat.

If you think you can help solve the mango twig-tip dieback mystery, delve into the published literature below!

Mango twig-tip dieback papers

Impact of Twig-Tip Dieback on Leaf Nutrient Status and Resorption Efficiency of Mango (Mangifera indica L.) Trees

• New research reveals how mango twig-tip dieback (MTTD) significantly alters mineral nutrition in Kensington Pride mango trees. The study found that trees with high MTTD infection had lower concentrations of several key nutrients and greater overall nutrient imbalances compared to less infected trees. These findings highlight the importance of targeted nutrient management strategies in MTTD-affected orchards and open new avenues for understanding and combating this emerging threat to mango production.

Electronic Nose and GC-MS Analysis to Detect Mango Twig Tip Dieback in Mango (Mangifera indica) and Panama Disease (TR4) in Banana (Musa acuminata)

• This study compares three E-nose devices against traditional gas chromatography-mass spectrometry, revealing their potential for rapid, non-invasive diagnosis of mango twig tip dieback and Panama disease. This innovative approach could transform agricultural biosecurity and plant health monitoring, paving the way for more efficient disease management in large-scale farming operations.