Jun 14, 2023Sanitation and Summer Whitefly Management (2023)To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
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.
For those of you interested in the latest thoughts on ag technology, you might want to check out the keynote address given by John Deere1 at CES 2023 earlier this week. The presentation can be found on YouTube at: https://youtu.be/1kjZMHZl538. As you might expect, there is a fair amount of company self-promotion, but it is interesting to hear their view on the current state and future of agricultural technologies. A couple of key takeaways for me were that telematics and computer vision systems will play key roles in the future and that increased precision will continue to be a focus.
 Reference 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.
Growers and PCAs asked about the potential injury that can be caused by a mix of RoNeet (Cycloate) with Dual Magnum (S-Metholachlor) to spinach. We did a small evaluation including the treatments suggested by our friends to collect data. A 1.33pt rate for Dual M and 4.0 pt of RoNeet was sprayed for this trial. The lower Dual M recommended rate by the SLN for AZ in 2017 of 0.33 - 0.67 pt per acre was not included in the trial1. Here’s the treatment list:
Plots consisted of two rows 30ft long replicated four times with 10 seed lines, and the test was established in a randomized complete block design with four replications. All treatments were applied preemergence. A CO2 backpack with a 4 flat fan nozzle boom spaced at 20” was used delivering 20 gallons/acre.
Planting was done on Nov 17, 2022, then the next day the treatments were applied and incorporated immediately with sprinkler irrigation. Crop injury was evaluated on December 13, 21 and 27. The combination of RoNeet plus S-Metolachlor at the rates above mentioned presented the highest phytotoxicity symptoms in the form of chlorosis, stunting, and reduced stand. Also, Dual Magnum alone at the rate of 1.33 pt/a presented injury in lower proportion compared to the combination with RoNeet. In this evaluation the 4pt/a rate of RoNeet did not present injury to the spinach as you can see comparing with the untreated plots below. Weed control data was not collected due to inconsistent and low populations. Only phytotoxicity was evaluated.
The most representative images of the plots are below:
Figure 1. Preemergence herbicide injury evaluation on spinach