May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
Clovers can be very difficult to control weeds here, but it is also a major crop and common ornamental. Clovers can survive under poor growing conditions and are not controlled with glyphosate and seem to get worse every year. There are more than 50 types and 300 species of clover and they can be easily misidentified. They are all in the legume (Fabracea) family and can use a bacterium (rhizobium) in the soil to convert nitrogen in the atmosphere to a form that they and other plants can use for fertilizer. There are only 4 or 5 clover species that are agricultural pests here. The ones we get the most questions on are white and yellow sweet clover. These are in the Melilotus family. White sweet clover (Melilotus albus) is tall for a clover and can get 3 to 5 foot in height. The leaves are thinner than most clovers and this difficult to control weed lives at least 2 years and sometimes longer. Glyphosate and most of the contact herbicides do not control it. The plant growth regulator herbicides work best. Yellow sweet clover (Melilotus officinalis) is less common here. The flowers are yellow, and it is not as tall and vegetative as white sweet clover. Yellow is more common at higher elevations. California burclover (Medicago polymorpha) and Black medic (Medicago lupina) are in the same genus as alfalfa and are more of a problem in landscapes, parks and golf courses than in agricultural fields here. They do not grow upright and spread below the crop or turf. The true clovers are in the Trifolium genus and include white and strawberry clover. These creep along the ground and root at the nodes of the stem. These are more of a urban landscape weed and not considered an agricultural problem. Creeping woodsorrel or Oxyalis looks like a clover but it is not related. It is a turf weed that spreads rapidly along the ground and can live for several years. Preemergent herbicides are effective against all these clovers before they become established. The postemergence herbicides that are most effective in controlling these clovers are the plant growth regulators. Contact herbicides and glyphosate are generally ineffective.
Last year we had a lot of watermelon fields infected with Fusarium from Winterhaven to Yuma, Wellton, and Mohawk Valley. Rain, and overwatering of fields when plants set fruits might have contributed to the disease development.
Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum, is one of the oldest described Fusarium wilt diseases and the most economically important disease of watermelon worldwide. It occurs on every continent except Antarctica and new races of the pathogen continue to impact production in many areas around the world. Long-term survival of the pathogen in the soil and the evolution of new races make management of Fusarium wilt difficult.
Symptoms of Fusarium can sometimes be confused with water deficiency, even though there is plenty of water in the field. In Yuma valley we have seen fusarium problem in some overwatered fields.
Initial symptoms often include a dull, gray green appearance of leaves that precedes a loss of turgor pressure and wilting. Wilting is followed by a yellowing of the leaves and finally necrosis. The wilting generally starts with the older leaves and progresses to the younger foliage. Under conditions of high inoculum density or a very susceptible host, the entire plant may wilt and die within a short time. Affected plants that do not die are often stunted and have considerably reduced yields. Under high inoculum pressure, seedlings may damp off as they emerge from the soil.
Initial infection of seedlings usually occurs from chlamydospores (resting structure) that have overwintered in the soil. Chlamydospores germinate and produce infection hyphae that penetrate the root cortex, often where the lateral roots emerge. Infection may be enhanced by wounds or damage to the roots. The fungus colonizes the root cortex and soon invades the xylem tissue, where it produces more mycelia and microconidia. Consequently, the fungus becomes systemic and often can be isolated from tissue well away from the roots. The vascular damage we see in the roots is the defense mechanism of the plant to impede the movement of pathogen.
Disease management include planting clean seeds/transplants, use of resistant cultivars, crop rotation, soil fumigation, soil solarization, grafting, biological control. An integrated approach utilizing two or more methods is required for successful disease management.
Vegetable season is rapidly approaching, and it won’t be long before established stands of lettuce are ready to be thinned. Since their commercialization about 8 years ago, use automated lettuce thinning machines has become a common practice in the Yuma area. These machines intermittently spray solutions of herbicide or fertilizer in the plant row to kill excess seedlings. A question growers might have is whether or not automated lettuce thinning is also effective at controlling in-row weeds. Research studies have shown it is.
In trials conducted in 2014 and 2015, Mosqueda et al. (2017) compared the performance of four different automated thinning machines with hand thinning. Eight commercial field sites near Salinas, CA were utilized in the study. Results showed that automated thinning and hand thinning removed 68% and 73% of the weeds respectively (percentage data provided in Smith, 2015). The differences were not statistically significant, implying that automated thinning was as effective at in-row weed removal as compared to hand thinning.
It’s interesting to note that both thinning methods controlled essentially all in-row weeds except those close to the “keeper” crop plant. To make this conclusion, one can estimate the percentage of the seed line that was sprayed or hoed by using the target plant spacing of 11” and assuming that a 1.5” safety/buffer distance was used on either side of the keeper crop plant. Given these parameters, the percentage of in-row area that was not sprayed or hoed out can be calculated as ((1.5 + 1.5)/11 x 100), or 27%. The sprayed or hoed out area is 100% minus this value, or 73%. This value compares very favorably to the 68-73% weed control efficacy found by Mosqueda et al. (2017).
Automated thinner manufacturers offer an option where spray is also applied to the sides of the keeper plant relative to the direction of travel. The concept is to control as many weed as possible near the crop plant. An example of the spray pattern described is shown in Fig. 1. Given the demonstrated high efficacy of spray solutions used for automated thinning, this technique may be an option worth considering for the upcoming growing season as weeds close to the crop plant are the most difficult and time consuming to control.
Mosqueda, E., Smith, R., Goorahoo, D. & Shrestha, A. Automated lettuce thinners reduce labor requirements and increase speed of thinning. California Agriculture, 72(2):114-119.
Smith, R. Impact of automated thinners on weeds and lettuce production. In Proc. 67th Annual California Weed Science Society 67: 44. Salinas, Calif.: California Weed Science Society.
Beet armyworm: Moth activity has declined in most trap location and remains below average for this point in the season.
Cabbage looper: Cabbage looper activity remains unusually low for early November. Trap catches picked up a bit in Wellton and Gila Valley.
Whitefly: Adult movement has been about average for this time of year. Activity highest in Wellton near fall melons being harvested.
Thrips: Thrips activity picking up significantly in several trap locations; activity increased significantly in Bard and Dome and Gila Valleys.
Aphids: Aphid numbers peaked last week in many locations, particularly in the Gilas and Yuma Valleys.
Leafminers: Adult activity increased significantly last week, particularly in the Wellton area near melons.