Citrus trees are also affected by high temperatures
In a small rural agricultural community in Paraguay, something unsettling has been happening to the citrus trees. At the local school, students walk up the director and ask permission to eat mandarin oranges from the trees (he deems if they are ripe enough to pick, and the kids are trained well). They gather at the tree near the school garden and scavenge for specimens that are healthy. Many have fallen prematurely and sit rotting on the ground, something that didn’t used to happen. The orange tree on the school property is now overlooked, considered useless due to disease. Some community members are facing the same problems. One woman in the community mentioned to me that ten years ago, citrus disease was not a big problem. Other neighbors talk about how trees that used to have fruit until August are now barren by the end of June due to fruit drop. Although the market has been poor, citrus fruits have been a source of income for families in this community, one that now appears to be threatened. Not to mention the hordes of children who like to gather around the tangerine (mandarina) trees to pick a sweet treat.Could it be that climate change is causing tree disease here? Some community members believe that drifting chemicals from soy plantations are causing this and other problems (small chickens are also dropping dead in large numbers, for example). Carrying a sample of leaves and pictures of a diseased tree, I traveled to IPTA (Instituto Paraguayo de Tecnologia Agraria) in Caacupe. One of the citrus specialists identified several sicknesses, one of which being anthracnose, which can cause premature fruit rot and drop. Anthracnose is caused by fungi of the genus Colletotrichum (gloesporoides and acutatum). Research has shown that the presence of Colletotrichum acutatum augments with increased temperatures (R. Ghini et al. 2011). Other studies have shown that fecundity of the fungus could increase and cause quicker pathogen evolution in a high CO2 environment (New Phytologist 2003). Some trees in the community have also dried up, losing all their leaves. This is caused by mites according to specialists in Paraguay (IPTA 2017), and research shows that citrus mite populations also increase with increasing temperature (Abu Bakar et al. 2016). Citrus black spot disease also causes premature fruit drop and is expected to increase with increased temperatures (Dewdney, Ghini et. al 2011). It has been found in Argentina and Brazil (Krueger 2016). So yes, it appears as though climate could make a difference in the availability of healthy citrus.
My visit to IPTA revealed that certain chemical products are prescribed for treating diseases. Specialists recognized that the need for chemicals has increased due to a decrease in natural enemies and ecosystems, as well as decreased soil fertility. The community mentioned above is located in what used to be all Atlantic Forest, and deforestation has changed the landscape significantly in the last fifteen years. According to the employees at IPTA, Paraguayans are still used to the limited management needs of the past, when the reality is that chemicals and more intensive management have become more important for citrus production as the environment changes.
Temperatures in Paraguay have been increasing in general, something community members have noticed. The school director talked to me about his days of youth, when by May it was already cold, around seven or ten degrees Celsius. This year, May temperatures have been in the 20s, even getting close to 30. He has also seen a change in the intensity of storms. At one point citrus trees were planted to protect banana trees from heavy winds. Lately, banana stalks have been falling during heavy storms despite this protection.
In the past in this community, cotton was grown as a source of income. At some point the crop stopped producing well, and farmers switched to other crops. Interestingly enough, anthracnose has also been a recorded disease on cotton in Paraguay (Katusbe and Romero 1991, M. E. Salustiano et al. 2014).
In an area as heavily deforested as the Atlantic Forest region of Paraguay, ecosystem and climate changes are both contributing to a shift in the way agriculture is managed. Adaptation has involved alterations in crops and increases in chemical use. If climate change can indeed be identified as a factor in problems with plant disease, we can only expect more to come.
(2003) “Climate change and plant pathosystems – future disease prevention starts here.” New Phytologist 159: 531-538.
Abu Baker, M. (2016) “Influence of weather factors on the seasonal abundance of citrus mite Eutetranychus orientalis (Klein) on different citrus cultivars.” Journal of Entomology and Zoology Studies 4 (1): 105-111.
Dewdney, Megan. “Fungal Diseases of Citrus Fruit and Foilage.” Presentation.
Ghini, R., Bettiol, W., and E. Hamada (2011) “Diseases in tropical and plantation crops as affected by climate changes: current knowledge and perspectives.” Plant Pathology 60: 122-132.
Interviews, Instituto Paraguayo de Tecnologia Agraria. May 2017.
Katusbe, T. and M.I. Romero (1991) “Diseases of Major Crops in Paraguay.” JARQ 25: 172-175
Krueger, Robert R. (2016) “Emerging Non-HLB Citrus Threats and Resources for Management.” Maricopa County Citrus Seminar.
Salustiano, M.E., Marina, N.R., Abreu, L.M., da Silva Costa, S., da Cruz Machado, J., and L.H. Pfenning (2014) “The etiological agent of cotton ramulosis represents a single phylogenetic lineage within the Colletotrichum gloeosporioides species complex.” Tropical Plant Pathology 39 (5): 357-367.