May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
Sprangletop has become increasingly widespread in Arizona mostly because of its growth habits and tolerance to many commonly used herbicides. It is in the Leptochloa genus which is derived from the Greek words leptos (thin) and chloa (grass). There are more than 150 species of sprangletop worldwide but only three in Arizona and two in Yuma County. The two that are the most common in the low desert are Mexican Sprangletop, which is Leptochloa uninervia and Red Sprangletop, Leptochloa filiformis. A third species, Bearded Sprangletop, Leptochloa fascicularis, is more common at higher elevations of 1500 feet or higher. It is not uncommon to find both Red and Mexican Sprangletop in the same field and it is not hard to distinguish them when they are side by side. Red Sprangletop has a light green leaf blade which is similar in width to watergrass and barnyardgrass. It has very fine hairs and very small and fine branches and spiklets. It also has a long membranous ligule. The name Red refers to the leaf sheath, which is characteristically red, rather than the seed head. Mexican Sprangletop has a thinner leaf blade which is darker green or grayish in color and similar in appearance to common bermudagrass. The seed head is distinctly coarser than that of Red Sprangletop. Side by side, leaf color and size of the seed make it easy to distinguish these two. Both of these grasses are classified as summer annuals, but they grow more like perennials in the low desert. Sprangletop does very well in the hottest part of the summer and typically germinates from seed during the hottest period between July and September. Once established, however, it often survives through the cold winter months. It grows into clumps that often appear to be dead during the winter. New shoots commonly grow from these established crowns the next season. When this occurs, preemergent herbicides such as Trifluralin or Prowl are ineffective. Some Sprangletop plants stay green and grow through the winter. Many of the postemergence, grass specific herbicides that control many grasses are ineffective on Sprangletop. This also has contributed to the spread of these weeds. Sethoxydim (Poast) and Fluazifop (Fusilade) do not control either Red or Mexican sprangletop. Only Clethodim (Select Max, Select, Arrow and others) is the only one of these grass herbicides that is effective and only at the highest labeled rates. Two applications are often necessary to achieve season long control.
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.
Weed escapes are easy to spot in vegetable fields at harvest time. Some growers have these weeds pulled, bagged and removed by hand from the field because they are unsightly and to reduce seedbank loads. This can be a costly operation. An alternative solution might be to use high voltage electricity to kill these weeds. The idea of using electricity to “zap” weeds is not new. Machines for agriculture applications were developed decades ago and commercially available in the late 1970’s. Although the devices worked, they were not widely adopted due in part to the availability of low cost and efficacious herbicides.
Because of environmental concerns, herbicide resistant weed issues and increased organic production, non-chemical, high voltage weed control technology is seeing a resurgence. There are now five companies, three established within the last four years, offering or developing machines for commercial agriculture. Although configurations differ, all machines operate using the same principles. To explain, consider the example of the machine shown in Fig. 1. The unit comprises high voltage electrodes (8-15 kV) positioned above the crop canopy, an electric generator and a soil engaging coulter connected to ground. During operation, when an electrode touches a weed protruding above the canopy, current flows through the plant back to the generator via the ground contacting coulter. Current flow combined with electrical resistance in the plant causes rapid heating and plant fluids to vaporize. This ruptures cell walls and kills the plant. Although there are few recent reports in the literature, prior research on dated machines showed that the technique can provide better than 98% weed control in moderate weed densities (15,000 weeds/acre) at travel speeds of 2 mph (Diprose & Benson, 1984).
Modern approaches that utilize high voltage electricity in combination with smart machines to spot treat weeds are being developed. The idea is to use camera imagery and artificial intelligence to locate weeds and high voltage electricity to kill them. One such machine being developed by the MASCOR Institute1 and the Zasso Group is an autonomous robot equipped with cameras, on-board computers and robotic arms (Fig. 2). As the machine moves through the field, high voltage electrodes mounted on the movable, computer controlled robotic arms zap weeds. Another unit is being developed by Stekettee and RootWave. It is tractor pulled and designed to travel at 3 mph. Stekettee’s machine vision system identifies the weeds and RootWave’s high voltage electric technology shocks the weed with a pulsed 5 kV charge. Power is supplied by a generator connected to the tractor’s PTO. Both systems are in late stages of development with field tests conducted in 2020.
These systems appear promising and if they prove to be effective and economical, may be something to look for in the future.
1Reference to a product or company is for specific information only and does not endorse or recommend that product or company to the exclusion of others that may be suitable.
Area wide Insect Trapping Network VegIPM Update, Vol. 11, No. 22, October 28, 2020
Results of pheromone and sticky trap catches can be viewed here.
Corn earworm: Moth activity is about average for mid-October, particularly in Dome Valley and south Yuma Valley.
Beet armyworm: Moths remain active throughout the desert, especially active in Tacna and Dome Valley.
Cabbage looper: Cabbage looper activity remains unusually low for early October, likely a result of unusually hot weather. Larvae are yet to show up in many fields.
Whitefly: Adult movement has been about average for this time of year. Activity highest in Tacna near fall melons.
Thrips: To date, thrips activity has been seasonably low at all trap locations; activity increased significantly in Roll.
Aphids: Aphids beginning to show in most traps along the Colorado River (Bard, Gila, Yuma) which is normal for this time of year. Recent high winds may begin to disperse them throughout the area.
Leafminers: Adult activity below normal for September, but high numbers caught in Wellton in areas where melon harvest has commenced.