Good Fruit Grower

July 1

Issue link:

Contents of this Issue


Page 33 of 47

Focus on soil health When there are nematode problems, nematodes are rarely the sole issue. by Geraldine Warner henever there is evidence of nematode problems in an orchard or vineyard— such as poor growth or yields— pathogens are usually working in concert with the nematodes, says Dr. Tom Forge, researcher with Agriculture and Agri-Food Canada at Agassiz, British Columbia. "They are rarely the sole issue." That's why Forge advises growers to focus on enhanc- ing soil health in order to address all the various aspects of the disease complex, not nematodes alone. Nematodes, which are microscopic roundworms, are the most abundant multicellular organisms on earth. There are more than 25,000 species. A cup of typical, healthy orchard or vineyard soil contains anywhere from 1,000 to 5,000 nematodes. The vast majority of those nematodes feed on bacteria and are involved in decomposition of dead plant material into simple nutrients, but there are also plant-parasitic nematodes in the mix, which are the ones of concern to growers. In certain soil conditions, the plant feeders can become predominant to the point of being damaging to the trees or vines. Nematode problems tend to be worse in poor, sandy soils where trees or vines are suffering water or nutrient stress. Nematode problems develop very slowly compared with problems caused by microbial plant pathogens, Forge said. It can take years before the symptoms become apparent above ground, especially with perennial, woody plants. Forge said scientists' understanding of the impacts of nematodes on trees is limited and tends to focus on replant situations because most experiments are done with young trees in pots, so that plants growing in soil with or without nematodes can be compared side by side. Nematodes are continually feeding on the root system. Consequently, they are a chronic drain on tree vigor, affecting tree health over multiple years, which makes it difficult to make a direct link between high populations of nematodes in an established orchard at any given point in time and the degree of damage in older trees, Forge said. "It's a very complicated situation." Apple seedlings grown in soil infested with root-lesion nematodes (on the left) and in soil without nematodes (on the right). Tree fruit and grape growers don't have the option of taking out the crop and replanting the following year, and as the populations build up year by year, the stresses on the trees become magnified. Root-lesion nematodes Root-lesion nematodes (Pratylenchus species) are the most prevalent plant- parasitic nematodes in the world. These worm-shaped nematodes measure about two thirds of a millimeter long and are known as migratory endoparasites because they wander in and out of roots. They use their piercing and sucking mouth parts to pen- etrate root cells, and then live and reproduce inside roots, causing dark lesions. The blackened, necrotic areas of the roots are susceptible to invasion by opportunistic soil fungi, some of which might be pathogens. When nema- todes are present, these fungi, such as Cylindrocarpon, can more easily get into the root system. The root-lesion nematode P. penetrans is not a new problem in fruit production. In the 1930s, scientists in California demonstrated its effects on potted apple trees. During the 1940s, nematicides were introduced that pro- vided good nematode control and increased fruit yields. In the 1950s and 1960s, researchers demonstrated the detrimental effects of P. penetrans and developed popula- tion thresholds for young trees. They found that a popu- lation of more than 300 root-lesion nematodes in one liter of soil could reduce tree growth by 40 percent. In 1955, a survey in British Columbia's Okanagan Valley found P. penetrans in 40 percent of apple tree roots and 34 percent of cherry tree roots. In the 1980s, another survey revealed P. penetrans in 80 percent of orchard blocks, and almost a third of those orchards had popula- tions of more than 1,000 nematodes per liter. "Undoubt- edly, those orchards were suffering some degree of yield loss and growth suppression as a result of nematodes," Forge said. Studies were done on soil treatments, such as pasteur- ization and biocides, and the research highlighted the important role that nematodes play in replant disease, even though other organisms can also suppress growth in replanted orchards. Research was also done showing the efficacy of nematicides on established trees. Ecto-parasitic nematodes Ecto-parasitic nematodes attack roots from the out- 34 JULY 2012 GOOD FRUIT GROWER side, feeding on the epidermal cells, and are generally not as damaging as root-lesion nematodes. They include the dagger nematode, which is widespread. Dagger nema- todes in the Okanagan Valley are not very damaging in their own right, Forge said, but they are possible vectors Photo courtesy of tom forge

Articles in this issue

Links on this page

Archives of this issue

view archives of Good Fruit Grower - July 1