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.
Options for delivering pesticides in melons post emergence are generally limited to spraying due to their viney growth habit. Injecting soil applied pesticides sufficiently close to the root zone for effective uptake with knife injectors in not possible without clogging equipment or causing crop injury. Several years ago, we investigated the use of an alternative device for delivering liquid chemicals post emergence called a point injection system. Point injection systems were developed in the late 1980’s as a method for applying ag chemicals post emergence with minimal root damage and soil disturbance. These systems utilize hollow, pointed tips attached to a rotatable wheel to inject liquid products into the root zone at precise intervals and depths (Fig. 1). Point injection technology has been used primarily as a fertilizer applicator in wheat and corn, however the system also has potential to improve the efficacy and usefulness of soil applied pesticides since it can be used to apply product to mature plants without damaging plant roots or causing crop injury.
In the fall of 2014, in cooperation with Mark White, Bayer CropScience, we tested a point injection system with Sivanto in cantaloupe. Traditionally, the product is soil applied at planting. The objective of the project was to determine if the technology can be used to apply Sivanto after plants have emerged to extend the window when the pesticide can be soil applied. Treatments included applications of 1) Sivanto at planting, 2) Scorpion 35SL at planting, 3) Sivanto post-emergence, 4) Scorpion 35SL post-emergence and 5) a combination of Scorpion 35SL at planting plus Sivanto post-emergence (Table 1). The target pest in these trials was whitefly. Results of the trials in Table 1 showed that use of point injection significantly reduced adult whitefly populations by about 2/3rds 6 (DAT) (45 DAP) as compared to other treatments. At the end of the trial, 24 DAT (63 DAP) cucurbit yellow stunting disorder virus (CYSDV) was about 25% lower. This was a 1-year trial and so conclusions should be considered preliminary, but the results do show good promise for late season soil application of Sivanto.
Over the years, we have conducted trials with point injection systems applying fungicides in cotton (Siemens, 2016, unpublished data) and fertilizers in broccoli, iceberg lettuce and romaine lettuce with improved pesticide efficacy, nutrient use efficiency and higher crop yields as compared to conventional application methods (Siemens and Gayler, 2016). Point injection systems are commercially available from several manufacturers. The Ag Mechanization Extension Program at the Yuma Ag Center has a commercial 4-row unit that is available for field demos. If anyone is interested, please contact me.
Siemens, M.C. & Gayler. R.R. (2016). Alternative systems for cultivating and side dressing specialty crops for improved nitrogen use efficiency. ASABE paper No. 162456725, pp. 9. St. Joseph, Mich.: ASABE.
You can link article to here - https://arizona.pure.elsevier.com/en/publications/alternative-systems-for-cultivating-and-side-dressing-specialty-c