THE THIRD EDITION of the Compendium of Pea Diseases and Pests, as is typical of the well-designed, lavishly-illustrated, modestly-priced paperback pest management books published by the American Phytopathological Society (APS; St. Paul, MN), integrates scientific information and color photos helpful for growing healthy peas. In the 20 years between 2nd and 3rd editions, “the acreage of pea (Pisum sativum) production has increased tremendously because of rising demands for a healthy diet and sustainable production systems,” write book editors Robert Harveson, Julie Pasche, Lyndon Porter, Weidong Chen and Mary Burrows.
The pea Compendium target audience is “field workers, diagnosticians, extension specialists, agronomists, horticulturists, entomologists, home gardeners, and other agriculture and horticulture professionals and enthusiasts as well as plant pathologists.” The Compendium strives to be “the preeminent, go-to source of knowledge and reference for pea diseases and insect pests worldwide.”
Peas are a diverse group of thousands of cultivars or varieties and related wild species whose seed colors range from green and yellow to red. There are spring peas and winter peas; tall peas and dwarf peas; grain peas and forage peas; yellow peas and green peas; round peas and wrinkled peas, etc. Hence, a concise illustrated chart of different types of peas –some of the many Pisum sativum cultivars; wild varieties like Pisum fulvum— would be useful for future pea Compendiums.
Field peas are traditional animal forages, also useful in cover crop mixes, rotations and as green mulches to build healthy soils (e.g. adding organic matter & legume nitrogen naturally fixed from the atmosphere by root-zone rhizobia). Fresh market peas, typically harvested for seeds and pods or tender leaf tips, include many heirloom varieties no longer listed in seed catalogs.
Fresh peas from the pod and edible-pod peas such as snow peas and sugar snaps are enjoying a renaissance. But the real staple in terms of acreage and international commerce are dried peas. High in protein and nutrients (some medicinal), dried peas include whole peas, split peas and ground pea flour. Worldwide 17.7 million tons of peas are grown for human and animal consumption on 9.2 million ha (22.7 million acres) of land, led by Canada, Russia, China, India, Ukraine, the USA, Australia, Ethiopia and Tanzania.
Many pea vine varieties are valued specifically for canning or freezing. “Clarence Birdseye, in the 1920s, after spending time in Canada with the native Intuit Eskimos, developed a method to flash freeze vegetables, thus providing fresh produce year-round for those consumers living far from production areas,” notes the Compendium. “Fortunately for him, peas just happen to be among the most effective vegetables for retaining natural flavor and color” when flash frozen. The Birds Eye label lives on in supermarket freezer sections.
“One of the eight Neolithic founder crops thought to be responsible for the origins of modern agriculture,” the pea “was first cultivated over 9,000 years ago,” and likely had had four diverse centers of origin, notes the Compendium introductory chapter, “Origin, Domestication, and History of Pea Production and Usage”. Carbonized remains from archaeological sites suggest “pea plants were domesticated in the Fertile Crescent of Southwest Asia, possibly northwestern India, Pakistan, or adjacent areas of the former Soviet Union and Afghanistan. Wild and primitive forms were found in ecologically diverse sites stretching from the Mediterranean region to Afghanistan and into the highlands of Ethiopia.”
From 1856 to 1863, eastern European monk/scientist Gregor Mendel utilized common garden peas, Pisum sativum, for the statistical inheritance experiments underlying modern genetics and breeding. Today’s plant breeders are studying the wild yellow pea, Pisum fulvum, which has scarlet flowers and good yields of red-yellow to black or brown seeds. Wild yellow peas grow wild in the eastern Mediterranean. Sixth and seventh millennium BC archaeological sites indicate it was eaten around what is now Turkey, Iraq and Israel. Pisum fulvum is a genetic source for breeding farm and garden peas with resistance to saline soils; fungal diseases such as Ascochyta blight, Fusarium wilts, pea rust and powdery mildew; and insect pests such as pea aphid and pea weevil.
Pisum fulvum also contains lectins (carbohydrate-binding proteins) that attack cancer cells, causing cell death (apoptosis). Similar compounds from cultivated Pisum sativum varieties also offer the potential advantage of fewer noxious side effects than current chemo-therapies. “In the last two decades, some plant lectins have been used to differentiate between malignant and benign tumours, and the degree of glycosylation was associated with the cancer metastasis; recently, the plant lectins have been developed in sophisticated microarrays for a better understanding of the malignant tumours, for diagnosis and identification of the different cancer developmental stages,” note Yassin et al. (Rev Chim-Bucharest, 2019).
Cultivated peas are classified as genus Pisum, species sativum. The word sativum refers to health-promoting or medicinal properties. “Pea seed is a rich source of high-quality protein; dietary fiber; low-fat, slowly digestible carbohydrates; minerals (iron, zinc, calcium, and magnesium); many vitamins (niacin, riboflavin, thiamine, B6, and folate); and a myriad of phytochemicals (e.g. polyphenols and saponins),” notes the APS pea Compendium. “Phytochemicals in pea have newly recognized health benefits and serve as antioxidants, anticarcinogens, and hypocholesterolemic agents. Galactose-oligosaccharides in pea promote gut health.”
“Pea seed protein concentration in cultivated pea ranges from 21 to 25%,” with 39.7% the upper limit known, says the Compendium. “Pea protein is rich in amino acids, lysine, and tryptophan but low in the sulfur-containing amino acids methionine and cysteine…Before 1700, it was unusual for peas to be consumed in any fashion other than as dried, mature seeds. The green immature cultivars were not developed for another 50-60 years by Knight. Since dry peas are difficult to completely cook without disintegration, the dried product was most often boiled for long periods until the starchy seeds fell apart, making a smooth but thick soup. Thus, pea soup was the standard method for use in the ancient world. This traditional practice provided a hearty, nutritious source of food during the winter months because the dried seeds stored so well…In Britain by the 1400s, it was one of the major crops grown and was so common that the words ‘pottage’ and ‘porridge’ meant peas. In fact, according to food historians, the English King John in the thirteenth century is thought to have died from overconsumption of peas with a reputed seven bowls at a single sitting.”
In randomized controlled experiments, pea fiber fights constipation. “Foods made with whole yellow pea flour reduced postprandial glucose responses in individuals and, thus, may have a role in the management of type 2 diabetes,” according to modern researchers. In Switzerland, cosmetic researchers are evaluating pea sprout extract topical applications and food supplements to stimulate hair growth and reduce hair loss. Another line of research is using antimicrobial phenolic extracts from sprouted peas to fight the microbe Helicobacter pylori (associated with gastric and duodenal ulcers) without the unwanted side-effects of conventional treatments. Early results indicate pea sprout extracts provide dose-dependent inhibition of H. Pylori, write Ho et al. in the Journal of Food Biochemistry (2006). Numerous scientific papers from around the world explore garden or field pea extracts and isolated compounds to combat human cancers.
It all starts in the soil, and the APS pea Compendium is especially strong when it comes to soil fungi (including oomycetes) attacking peas (e.g. root rots). Resistant varieties are a major bulwark against soil pathogens, but not always available. Oats or mustard family (Brassicaceae) plants planted prior to peas or as cover crops may help suppress pea root pathogens. Organic pea growers are exploring soil pathogen biological control with beneficial microbes such as Trichoderma, Pseudomonas, Streptomyces, and Bacillus spp. In small areas, seed priming is effective against damping-off diseases such as Pythium and Rhizoctonia.
HortScience (Nov. 2020) described intercropping organic snow peas and cherry tomatoes under polyethylene high tunnels. High tunnels have walls that can be rolled up or down to adjust the temperature, let in pollinators, exclude insects, etc. Sort of like a field greenhouse. Since snow peas are legumes supplying their own nitrogen and mature early, the Ontario, Canada organic grower, who prepared the soil with cover crops of buckwheat, sorghum and red clover, got good yields of both crops.
Greenhouses and high tunnels are growing in use, permitting crop production in seasons and climates otherwise adverse; also allowing early or later harvests when crops are in high demand and fetch good prices. Light reflected or filtered through greenhouse walls and supplemental lighting affect the light wavelengths & intensities reaching plants and arthropods. For example, ratios and intensities of red, blue, ultraviolet and other light wavelengths (colors) may affect and alter the survival, reproduction and growth of crop plants, plant pathogens, insects, spider mite and beneficial organisms. CABI recently published a whole book, based on an international symposium, discussing the manipulation of light for integrated pest management (IPM) of insects, spider mites and plant pathogens. Indeed, short bursts of selected light wavelengths are used in some commercial greenhouses to kill plant pathogens such as powdery mildew.
Beneficial soil microbes deserve more prominence in the Compendium, as they affect and can be part of pest management. Nitrogen-fixing bacteria (e.g. family Rhizobiaceae) colonizing the roots of peas and other legumes are famous for naturally supplying nitrogen, an often expensive fertilizer commodity. Nitrogen-fixing and Plant Growth Promoting Bacteria (PGPB) may be thought of as “beneficial root infections” or “good plant diseases”. Commercial growers, led by those practicing organic methods, are at the leading edge using PGPBs and microbial biofertilizers which may also provide biological control of plant pathogens and soil nematodes.
Some PGPBs supply plant growth hormones and increase the solubility and availability of soil phosphorous. Ranked #2 to nitrogen in crop growth importance and deficient in 40% of world soils, soil phosphorus often requires microbial actions (e.g. the symbiotic association of plant roots with arbuscular mycorrhizal fungi) to become available to plants. Bødker et al. (1998) showed that microbes associated with pea roots –specifically the arbuscular mycorrhizal fungus (AMF), Glomus intraradices— and inorganic phosphate result in lower levels of pea root diseases such as Aphanomyces euteiches.
AMF modes of action include inducing systemic resistance to plant pathogens in pea seedlings. Thus, future Compendium editions would benefit having a section on these beneficial soil microbes, similar to the excellent section on beneficial insects providing biological control of pea insect pests. To that insect gallery, I would add pea leafminers and beneficial predatory ground beetles and tiger beetles whose numbers are boosted when grains or cereals are part of the cropping mix.
Suppressive soils that stop soilborne plant pathogens and nematodes need more research, and would make a good chapter or subheading. Naturally suppressive soils are not uncommon. Heyman et al. (2007) demonstrated experimentally that calcium soil amendments help create soils naturally suppressive to Aphanomyces root rot of pea. In greenhouse experiments, Williams-Woodward et al. (2020) noted that the oat cultivar ‘Troy’ turned into soils as a green manure provided better pea root rot control than a fallow period. Mustards, cabbage, rape and similar Cruciferous crops contain glucosinolates (mustard gas chemicals) suppressing soil pests. In Columbia, South America soils, a Trichoderma biocontrol microbe made soils suppressive to Rhizoctonia root rot of peas and other crops (Chet & Baker, 1981).
Interestingly, in greenhouse experiments pea seed powder acts as a botanical herbicide, and suppresses broadleaf weeds and grassy weeds in wheat. There are also scientific reports of pea root-zone (allelopathic) chemicals suppressing weeds in cereal grains and other crops. There are also reports that peas secrete root chemicals suppressing subsequent pea crops. The Compendium mentions peas as a rotation crop for cereals like wheat, corn, millet and barley.
For those interested in companion planting: Cucumber and carrot seed extracts have a positive effect on pea seedling growth; onion and garlic seed extracts negatively impact pea seedling growth. In the 1930s, a Tennessee entomologist named Marcovitch designed one of the early modern scientific farm companion planting experiments. Turnip strips were planted adjacent to strips of peas and other crops in March. Lady beetles, small parasitic wasps and other natural enemies migrated from the turnips to destroy early-season aphid populations in adjacent rows of peas, beans, corn, okra, cotton, cucumbers and watermelons. Watermelons far from turnip rows were destroyed by early season aphids.
Although peas have been cultivated for thousands of years, we do not have a definitive scientific answer to whether we can plant peas over and over again on the same ground. Clearly, plant pathologists can play a role in answering the question. If peas are anything like wheat, carnations and other crops, then growing them continuously in the same soil might lead to the buildup of biological control organisms (e.g. Trichoderma spp.) that make soils naturally suppressive or resistant to soil pathogens. Or if peas are different, knowing why some plants grown successively year after year cannot produce suppressive soils would be a valuable research outcome.
Biocontrol Beat 