Allelopathy in weeds and crops: myths and facts

Jed Colquhoun Department of Horticulture in weeds and crops: outline

• What is allelopathy? • Field observations of allelopathy • Practical uses for weed management • Can we put the sprayer away? What is allelopathy?

• The effect of one plant on another plant through release of a chemical compound into the environment – “Plant-produced ”

• Greek derivation: “allelo” and “pathos” = mutual suffering Allelopathy is not new

• Theophrastus, 300 B.C.: chickpea allelopathic to weeds

• Plinus Secudus, 1 A.D.: corn “scorched” by chickpea, barley, and bitter vetch

• Juglone and the tree Field observations attributed to allelopathy Field observations attributed to allelopathy Myth vs. Fact

• Allelopathy is very difficult to isolate: – Weed suppression = allelopathy + physical interference – Physical interference includes impedance of light, water, nutrients by cover crop residue or living mulch Myth vs. Fact

• Allelopathy research is difficult: – Greenhouse research: doesn’t account for effect of microorganisms, plant stress, weather, soil type, growth stage – Field research: difficult to separate physical interference from allelopathy – Difficult to isolate complex allelopathic compounds Crops can be allelopathic to weeds

• Three methods for using allelopathy in weed management: – As a winter cover crop, with residue providing allelopathic compounds – As a living mulch during the cropping season –As an isolated compound from an allelopathic plant, applied as an Crops can be allelopathic to weeds: examples

• Rye as an allelopathic cover crop in field studies –Reduced common ragweed (43%), green foxtail (80%), redroot pigweed (95%), and common purslane (100%) – Physical interference not separated from allelopathy Crops can be allelopathic to weeds: examples

• Rye as an allelopathic cover crop – Residue leached of allelopathic compounds compared to residue with allelopathic compounds

– Eastern black nightshade and yellow foxtail suppression did not differ between leached and non-leached residue

– The layer of rye residue on the soil surface accounts for majority of suppression

– Barley and crimson clover were allelopathic Crops can be allelopathic to weeds: examples

• Oat cultivars differ in allelopathy 24 oat cultivars grown in greenhouse

Oat residue soaked for 5 hours in water

Common lambsquarters seed germinated with water from oat soaking

Germination quantified Crops can be allelopathic to weeds: examples

• Oat cultivars differ in allelopathy – 20 of 24 cultivars reduced common lambsquarters germination – ‘Ruby’ cultivar reduced germination by 86% • Minimal selectivity: pea germination also reduced by 71% Crops can be allelopathic to weeds: examples

• Rape-turnip is allelopathic to several weeds – Isolated compounds reduced spiny sowthistle, mayweed, smooth pigweed, and barnyardgrass germination – At higher concentrations, germination was completely eliminated Weeds can be allelopathic to the crop

• Over 240 allelopathic weed species documented • Velvetleaf: radish, corn, , tomato • Common lambsquarters: corn, cucumber, oat, soybean, tomato, wheat • Redroot pigweed: barley, cabbage, carrot, cauliflower, corn, cotton, eggplant, pepper, squash, soybean, tobacco, tomato, wheat Allelopathic plants as natural herbicide sources

• Synthetic herbicides based on natural plant compounds •Advantages:new target sites, water soluble, perceived as more environmentally friendly • Disadvantages: chemically complex, difficult to isolate and produce, not stable – Example: sorgoleone, from sorghum, inhibits photosynthesis better than , but is short-lived and not stable Allelopathic plants as natural herbicide sources

•Callisto () – Derived from a compound () isolated from the callistemon (bottle brush) plant

Source: www.syngenta.com Allelopathic plants as natural herbicide sources

• Rely, Liberty () – Derived from microbes – Synthetic version of phosphinothricin, a breakdown of a compound produced by Streptomyces spp. Can we put the sprayer away?

• Allelopathic compounds: – Short-lived in soil – Complex and unpredictable – Non-selective – Expensive to synthesize – Potential mammalian , carcinogenic, allergenic Can we put the sprayer away?

• Over 2,900 papers published on allelopathy, dating back to 300 B.C. • Despite allelopathic crops, weeds are still problematic – Breed for allelopathy? • Allelopathy is another tool in an integrated program, but is not a silver bullet