May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
Pigweeds are some of the most common summer annual broadleaf weeds in the low deserts. Although they are often lumped together, there are 4 different species of pigweed that are common here and more than 10 species that occur as weeds in California and Arizona. Their growth habits and response to herbicides are similar. It is easy to identify them by physical characteristics but one species of pigweed can hybridize with another and become less distinguishable.
Palmer Amaranth (Amaranthus palmeri) is probably the most common pigweed species found in this region. It is very aggressive and fast growing and can become 6 feet tall or higher if uncontrolled. It has one thick stem and several lateral branches. The leaves are lance shaped, hairless and have distinctive white veins on the underside. It has flowering tassels that become stiff and spiny. This species has become resistant to Glyphosate in many parts of the county.
Redroot Pigweed (Amaranthus retroflexus) is probably the second most common pigweed species. It is shorter and the seed heads are smaller, in clusters and have stiff spine-like scales. It has leaf hairs on the margins and the veins are often reddish. The lower stems are often reddish. This species will hybridize with Palmer Amaranth and become less distinguishable.
Tumble Pigweed (Amaranthus albus) is very different from Palmers or Redroot. It grows lower to the ground and has many branches that turn upright. The leaves are much smaller and narrower. The numerous stems are light green rather than red. The seed heads are small, spiny and at the base of the leaves rather than in long terminal spikes. When mature, the branches are sticky, stiff bristles that break off at the ground and tumble with the wind.
Prostrate Pigweed (Amaranthus blitoides) is very similar to Tumble Pigweed but the stems are more prostrate, grow close to the ground and form mats. The stems and leaves are smaller and reddish rather than light green.
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.
Mark C. Siemens
Vol. 12, Issue 9, Published 5/5/2021
The Yuma County Leaf Wetness Network remains in place for the 2018/19 vegetable season. Growers and PCAs may access information generated by the network by entering the following internet address: http://188.8.131.52:460
Upon entering the address above, you will be transferred to internet page that provides a series of tabs at the top of the page. Simply click on the tabs to access the information of interest.
Corn earworm: First significant CEW moth activity since mid-November; particularly active in Dome/Wellton/Tacna areas.
Beet armyworm: Moth counts remain very low consistent with seasonal temperatures, but below average for this point in the season.
Cabbage looper: Slight increase in activity, but moth counts remain unusually low for this time of season.
Whitefly: Adult movement is at seasonal low consistent with temperatures and lack of melons or cotton.
Thrips: Thrips activity beginning to pick up, particularly in Tacna and Yuma Valley. Movement is still below average for February.
Aphids: Seasonal aphid counts peaked in early February and tending down last week. Counts remain high in Gila Valley and Wellton. Above average for this time of year.
Leafminers: Adult activity remains light in most trap locations. Trap counts increasing slightly in the South Gila Valley.