Conyza species: distribution and evolution of multiple target-site herbicide resistances.

MAIN CONCLUSION: Distribution of Conyza species is well correlated with human interference. Multiple herbicide resistance is caused by the attempt to overcome resistance to one mode of action by overuse of another. Conyza canadensis (CC) and Conyza bonariensis (CB) are troublesome weeds around the world. Extensive use of herbicides has led to the evolution of numerous Conyza spp. herbicide-resistant populations. Seeds of 91 CC and CB populations were collected across Israel. They were mostly found (86 %) in roadsides and urban habitats, two disturbed habitats that had been dramatically impacted by human activities, thus we classify these species as anthropogenic. Although pyrithiobac-sodium was only used in cotton fields, 90 % of Conyza spp. populations were identified as pyrithiobac-sodium resistant, suggesting possible natural resistance to pyrithiobac-sodium. CC21 and CC17 C. canadensis populations were highly resistant to all tested ALS inhibitors due to a substitution in the ALS gene from Trp574 to Leu. They were also atrazine resistant due to a substitution in the psbA gene from Ser264 to Gly. The high level of imazapyr and pyrithiobac-sodium resistance observed in the CC10 population was due to an Ala205 to Val substitution. However, high resistance to sulfometuron methyl and pyrithiobac-sodium in population CC6 was due to a point mutation at Pro197 to Ser. All resistant plants of CC21 population showed both psbA (Ser264 to Gly) and ALS (Trp574 to Leu) substitutions, leading us to the conclusion that the attempt to overcome resistance to one mode of action by overuse of another will most likely lead to multiple herbicide resistance. Furthermore, we concluded that only individuals that carry both mutations could survive the shift between the two modes of action and overcome the fitness cost associated with the PSII resistance.

Do Traits Travel? Multiple-Herbicide-Resistant A. tuberculatus, an Alien Weed Species in Israel.

Amaranthus tuberculatus is the most common weed in soybean and corn in the USA and Canada. In Israel, it has been a minor riverbank weed. However, in recent years, growing densities of this plant have been observed in field crops, orchards, and roadsides. Between 2017 and 2022, we surveyed the distribution of A. tuberculatus and collected seeds for further study. We identified three main distribution zones in Israel: the Jezreel Valley, Hula Valley, and Coastal Plain. Most of the populations were found near water sources, fishponds, barns, dairies, or bird-feeding sites, suggesting the involvement of imported grain in introducing A. tuberculatus to Israel. Populations were screened for their responses to various post-emergence herbicides (i.e., ALS, EPSPS, PPO, HPPD, and PSII inhibitors). Several populations from the Jezreel Valley were found to be putatively resistant to ALS, EPSPS, and PPO inhibitors. The responses of those populations to trifloxysulfuron, glyphosate, and carfentrazone-ethyl were also studied. A single ALS-, EPSPS- and PPO-resistant plant was vegetatively propagated to create a clonal population, which was treated with foramsulfuron, glyphosate, and carfentrazone-ethyl. No resistance to PSII or HPPD inhibitors was observed, but resistance to herbicides that inhibit ALS, EPSPS, and PPO was observed. A clonal propagation assay revealed the existence of a population that was resistant to ALS, EPSPS, and PPO inhibitors. Since the local A. tuberculatus populations have not been exposed to herbicide selection pressure, these traits probably reached Israel through seed-mediated gene flow via imported grain.

Imazapic Herbigation for Egyptian Broomrape (Phelipanche aegyptiaca) Control in Processing Tomatoes-Laboratory and Greenhouse Studies.

Parasitic plants belonging to the Orobanchaceae family include species that cause heavy damage to crops in Mediterranean climate regions. Phelipanche aegyptiaca is the most common of the Orobanchaceae species in Israel inflicting heavy damage to a wide range of broadleaf crops, including processing tomatoes. P. aegyptiaca is extremely difficult to control due to its minute and vast number of seeds and its underground association with host plant roots. The highly efficient attachment of the parasite haustoria into the host phloem and xylem enables the diversion of water, assimilates and minerals from the host into the parasite. Drip irrigation is the most common method of irrigation in processing tomatoes in Israel, but the delivery of herbicides via drip irrigation systems (herbigation) has not been thoroughly studied. The aim of these studies was to test, under laboratory and greenhouse conditions, the factors involved in the behavior of soil-herbigated imazapic, and the consequential influence of imazapic on P. aegyptiaca and tomato plants. Dose-response Petri dish studies showed that imazapic does not impede P. aegyptiaca seed germination and non-attached seedlings, even at the high rate of 5000 ppb. Imazapic applied to tomato roots inoculated with P. aegyptiaca seeds in a PE bag system revealed that the parasite is killed only after its attachment to the tomato roots, at concentrations as low as 2.5 ppb. Imazapic sorption curves and calculated Kd and Koc values indicated that the herbicide Kd is similar in all soils excluding a two-fold higher coefficient in the Gadash farm soil, while the Koc was similar in all soils except the Eden farm soil, in which it was more than twofold lower. In greenhouse studies, control of P. aegyptiaca was achieved at >2.5 ppb imazapic, but adequate control requires repeated applications due to the 7-day half-life (t1/2) of the herbicide in the soil. Tracking of imazapic in soil and tomato roots revealed that the herbicide accumulates in the tomato host plant roots, but its movement to newly formed roots is limited. The data obtained in the laboratory and greenhouse studies provide invaluable knowledge for devising field imazapic application strategies via drip irrigation systems for efficient and selective broomrape control.

Molecular basis for multiple resistance to acetolactate synthase-inhibiting herbicides and atrazine in Amaranthus blitoides (prostrate pigweed).

Amaranthus blitoides S. Watson (prostrate pigweed) populations resistant to acetolactate synthase (ALS; EC 4.1.3.18)-inhibiting herbicides and triazines (SuR/TR) were found in Israel. The Ganot population was 6- to 790-fold more resistant to ALS inhibitors than the wild type due to an altered target site. Molecular analyses showed that the Ganot population was a mixture of two biotypes: (i) SuRA/TR in which domain A of the als gene differed in one nucleotide, resulting in substitution of Pro by Ser 188; (ii) SuRB/TR in which a mutation in domain B led to a substitution of Trp by Leu 569. The mutation in domain A resulted in resistance to all ALS inhibitors except imidazolinones, whereas the mutation in domain B led to resistance to all ALS inhibitors tested. SuRA/TR and SuRB/TR are multiple-resistant with an additional single mutation in the plastidic psbA gene that changes Ser 264 to Gly in the D1 protein, leading to triazine resistance. It is evident that plants within a population exposed to a similar selection pressure may show different patterns of cross-resistance due to three different point mutations. This unique phenomenon renders planning of rational weed management difficult or even impossible.