Non synthetic, natural originated compounds

Some naturally sourced ingredients are allowed for herbicidal use. Currently, however, organic herbicides and herbicides with organic active ingredient play a minor role in the organic weed control. These include certain formulations of acetic acid (concentrated vinegar), Pelargonic acid, corn gluten meal, and essential oils.

Corn gluten meal is applied as a pre-emergent herbicide against crabgrass (Digitaria sp.) and other lawn weeds by inhibition of roots formation of weeds. Timing of application is crucial, because if weeds have already germinated and taken roots, corn gluten will serve as fertilizer. It also has nutritional properties with 10 percent nitrogen by weight, thus can be used as organic source of nitrogen. Corn gluten needs water just after application, but a dry period is then required in order to trigger inhibitor effects on root production. The first application will suppress only about 60% of the weed seeds, and a single application may help suppress weeds for 4 to 6 weeks. Heavy soils, extended rainy weather and hot spells may require a monthly application or a second application in late summer. After several applications, corn gluten sometimes reaches 80% effectiveness. Application rates vary by form: powder, pelletized or granulated. The standard application rate is 10 kg of corn gluten per 100 square meter of lawn. This rate also provides about 1 kg of nitrogen per 10 square meter. The effects of corn gluten are cumulative, meaning that the results improve with repeated use over time. The most prominent weed killing essential oils is clove (Syzygium aromaticum), that can be the only oil you apply as a natural weed killer spray. Wintergreen (Gaultheria fragrantissima), cinnamon (Cinnamomum verum) and summer savory (Satureja hortensis) can enhance the weed killer effect of clover.

A few selective herbicides with organic active ingredient based on fungal pathogens have also been developed consisting of phytotoxins, pathogens, and other microbes used as biological weed control. Herbicides with organic active ingredient may be compounds and secondary metabolites derived from microbes such as fungi, bacteria or protozoa; or phytotoxic plant residues, extracts or single compounds derived from other plant species. On a global scale, only thirteen herbicides with organic active ingredient derived from micro-organisms or natural molecules have been developed. Among the thirteen authorized herbicides of biological origin, nine are based on fungal microorganisms, three on bacterial micro-organisms, and one contains an active substance that is a natural plant extract (Table 5.2).

Table 5.2 Herbicides with organic active ingredient developed for weed management in ecological farming

Product name	Active agent	Weed	Registration	On market
De Vine®	strain MVW of the oomycete Phytophthora palmivora	strangler vine (Morrenia odorata)	1981, USA	unknown
Collego™ (LockDown)	spores of Colletotrichum gloeosporioides 20358 strain	northern jointvetch (Aeschynomene virginica)	1982/2006, USA	available
Collego™ (LockDown)	spores of Colletotrichum gloeosporioides 20358 strain	northern jointvetch (Aeschynomene virginica)	1982/2006, USA	available
BioMal®	Colletotrichum gloeosporioides f.sp. malvae	low mallow (Malva pusilla)	1992, Canada	available but production limited
Camperico®	Xanthomonas campestris strain JTP482	annual meadow grass (Poa annua)	1997, Japan	not available
Woad Warrior	fungus Puccinia thlaspeos	glastum (Isatis tinctoria)	2002, USA	not available
Chontrol®=Ecoclear®	Chondrostereum purpureum strain PFC 2139	shoots from black cherry (Prunus serotina) stumps canadian poplar (Populus euramericana) in the sandy soils of conifer forests	2004/2007	available
Mycotech™	Chondrostereum purpureum strain HQ1	shoots from black cherry (Prunus serotina) stumps canadian poplar (Populus euramericana) in the sandy soils of conifer forests	2004/2007 Canada	not available
Smoulder WP, Smoulder G	Alternaria destruens strain 059	hell-weed species (Cuscuta sp.)	2005, USA	available
Sarritor	Sclerotinia minor strain IMI 344141	dicot weeds in turf	2007, Canada	available
Organo-Sol® (Kona)	Lactobacillus casei strain LPT-111 L. rhamnosus strain LPT-21 L. lactis ssp. lactis strain LL64/CSL L. lactis ssp. lactis strain LL102/CSL L. lactis ssp. cremoris strain M11/CSL	white clover (Trifolium repens) red clover (Trifolium pratense) bird's-foot trefoil (Lotus corniculatus) black medick (Medicago lupulina) wood sorrel (Oxalis acetosella)	2010, Canada	available
Phoma	Phoma macrostoma strain 94-44B	dicots	2011, USA and Canada	available
Opportune™	thaxtomin A, a compound that is produced by fermentation from the Streptomyces acidiscabies strain RL-110	dandelion (Taraxacum officinale)	2012, USA	available
Beloukha®*	derived from rapeseed oil, using a natural extraction process (nonanoic acid and pelargonic acid)	grapevine to kill suckers and control weeds, and on potatoes to kill stems and leaves.	2015, USA	available

* Allowed in the EU

Herbicides with organic active ingredient could help increase both the efficacy of individual weed control techniques and the overall efficacy of the integrated weed management systems.

Herbal active ingredients

Many biologically active compounds are known to be produced by shoot plants. These compounds are secondary metabolites. Their biosynthesis can be derived from the metabolism of the primary compounds, i.e. they are only secondary in their biosynthesis and not in their significance. Secondary metabolites are end products that are synthesized from different materials in different metabolic pathways. Although attractive and repellent compounds are also found among them, the majority have an effect on living organisms mainly due to their inhibitory (toxic) nature. These secondary compounds can be biochemically diverse.

- thiophenes. Thiophenes are sulfur-containing aromatic compounds. Typical thiophenes are α-tertienyl and butene bitienyl. Both active ingredients can be found in our popular garden ornamental plant, the marigold species (Tagetes spp.). Thiophenes are likely to function as toxins in the plant-animal and plant-plant relationships, respectively. Thiophenes show a wide range of biological activity. They act primarily as phototoxins. In addition, their fungicidal, herbicidal and nematode effects are significant.

- coumarins. Coumarins are compounds made up of cinnamic acids. Their simplest structure is coumarin itself, but other coumarins (pyrano- and furanocoumarins) are also known. In plants, coumarins occur mainly as glycosides in sugar-like compounds. Physiologically extremely important compounds. Some coumarins (including coumarin itself) inhibit germination and cell elongation. One hundred times more effective growth inhibitors, such as phenolic acids used in practice.

- mono- and sesquiterpenes. Monoterpenes occur as essential oil components in the plant kingdom. They are found in the largest numbers in the family of lips, rutans and umbrellas. The synthesis of essential oils often takes place in specific cells or glandular hairs. It is known that glandular hairs on the leaf surface can produce and secrete essential oils. The function of essential oils differs from case to case. They have an inhibitory effect on germination and plant growth. For this reason, they are also important in the competition between plant species. This makes them suitable for weed control. Under laboratory conditions, the inhibitory effect of essential oils on the growth of bacteria and fungi was observed.

- triterpenes. Their glycosides are called saponins. Saponins are common in plants as complexes. For example, alfalfa, known as a forage plant, contains 11 saponins in addition to medicinal acid. They accumulate mainly in the leaves and fruits of the plant species concerned.

Table 5.3 Plant species and their parts can be applied in weed management as extract.

Plant	Typical picture of the genus or the species	Plant part applied	Active ingredient	Herbicide	Insecticide	Fungicide	Soil disinfectant
Tagetes sp.	Figure 5.2 (E. Takács)	flowering sprout	α-tertiaryl, butene bitienyl	+		+
Ranunculus sp.	Figure 5.3 (M. Ábele)	leaf shoot	ranunculin	+			+
Achillea sp.	Figure 5.4 (M. Ábele)	inflorescence, leaf	achillin, anacycline, procamazulene	+			+
Tanacetum vulgare	Figure 5.5 (M. Ábele)	flowering sprout	borneol, cineol, isothujon	+	+		+
Prunella sp.	Figure 5.6 (M. Ábele)	leaf shoot	ursolic acid	+
Centaurea sp.	Figure 5.7 (M. Ábele)	flowering sprout	centaurepenzin	+
Calendula officinalis	Figure 5.8 (shutterstock)	inflorescence	isorhamnetine	+	+
Aristolochia sp.	Figure 5.9 (M. Ábele)	fruit, rootstock	aristolochic acid	+			+
Mentha sp.	Figure 5.10 (M. Ábele)	leaf shoot	limonene, menthol, menton, mentofuran, pulegon	+		+	+
Artemisia sp.	Figure 5.11 (shutterstock)	leaf shoot	absinthin, bisabolene, artemisinin, thujon, cineol, tauremizine	+	+		+
Stachys annua	Figure 5.12 (M. Ábele)	flowering sprout	stachydrin	+
Salvia sp.	Figure 5.13 (M. Ábele)	leaf	cineol, cimol	+	+		+

Due to their short duration of action, plant extracts should be used in short-growing crops. Due to the relatively large amount of extract required to achieve the desired weed control effect, it is advisable to use plant extracts in a small area. In our opinion, plant extracts can be well integrated into the tools of organic farming with due care.