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Spinach is a member of the plant family Chenopodiaceae, which also includes crop plants such as beet and Swiss chard as well as weeds such as nettleleaf goosefoot and lambsquarters. Downy mildew can develop on all of these plants. Peronospora farinosa, the oomycete pathogen that causes the disease on this group of plants, exists as different races or subtypes. A spinach grower might wonder if isolates of the pathogen that cause downy mildew on beet, Swiss chard, or weed hosts can cause downy mildew on spinach. Former Monterey County, California Farm Advisor Steve Koike and colleagues conducted research to answer this question. Isolates of the pathogen from nettleleaf goosefoot, lambsquarters, beet and Swiss chard were each used to inoculate spinach; however, downy mildew did not develop. In a separate experiment, the downy mildew pathogen from spinach was used to inoculate nettleleaf goosefoot and lambsquarters. The result again was no disease development. The bottom line is that spinach growers need not worry about nettleleaf goosefoot, lambsquarters, beet or Swiss chard as potential sources of inoculum for their spinach crop. Even though the bluish-purple evidence of the pathogen on the underside of leaves is similar in appearance on all these plants, the particular isolates of the downy mildew pathogen have very specific and limited host ranges.
Cotton root rot, caused by Phymatotrichopsis omnivora, is the most destructive disease of dicotyledonous plants in Arizona. There are no known reliable control methods for this disease, and the difficulty in its management is most often directed at its survival deep in soils and its wide host range. Genetic diversity in P. omnivora and its potential role in disease are unknown. Isolating the fungus and reproducing the disease in the greenhouse or laboratory are problematic, making it difficult to assess the efficacy of potential treatments.
Nutrient deficiencies can be identified in field through visual observations. However, additional analysis, either plant or soil testing is often necessary to confirm nutrient stress. The following is a quick-reference flow chart that can be used in field to identify potential nutrient deficiencies. Each nutrient has unique deficiency symptoms. Deficiencies will appear in many ways. It can be confused with other plant problems like pathogens or spray damage. But nutrient deficiencies can often be recognized because they tend to form symmetrical patterns, where both sides of leaf or plant parts show the same pattern.