May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
Heavy and widespread infestations of common purslane come up during ground preparation for lettuce every year. This occurs in fields that were kept weed free the previous year and is difficult to understand.
There are probably several reasons for this.
Common Purslane is very prolific. It has been reported that one plant can produce up to 240,000 seeds. The stems are so succulent that plants can remain viable and make seed even after it is uprooted.
Once seed is mature it can be viable for as long as 40 years. It has very small, hard seed that can remain dormant in the soil for ss long as 40 years .So you may have to control weeds that got into the field a generation ago..
Multiple perennial germinations
Common Purslane is supposed to be a summer annual, but it germinates multiple times all year in the low desert. It takes 12 hours after receiving moisture in the summer and 7 days in the winter, but it keeps germinating. It has to be controlled when it is less than 2” in diameter. If you wait until most of it germinates the early plants will be too big. If you spray or cultivate when all the emerged plants are small you will miss many that have yet to emerge. It is best to treat early and control the later emerging plants with a selective herbicide.
When common purslane is broken in pieces it can reroot at the nodes. Late cultivation often spreads this weed. Cultivation is not a good option when purslane is larger than 2”. Herbicides are a better option on big plants.
Purslane has a very small light seed. It moves in irrigation water and blows in the wind. Even completely clean fields are likely is be reinfested by seeds that are carried by water and wind into the field.
Considering the above factors, the best option for controlling common purslane may be preirrigation to germinate the weeds and early herbicide application or cultivation . Kerb and Prefar are both good on purslane. Prefar should be used at planting to incorporate it with a lot of water and Kerb should be used later to avoid leaching but don’t wait too long and risk germination of the weeds. Purslane germinates from shallow depths and split applications of Kerb may be a good option.
It is about that time of the year/growing season when you start seeing bacterial diseases. With the rain we got last week and as plants get more vegetative growth bacterial issues become more prevalent. Cilantro and parsley are two crops grown in desert southwest that often suffer from bacterial leaf spots. Most times, the disease incidence is also high because of sprinkler irrigation used in these cropS. On both crops, initial symptoms of bacterial leaf spot are water-soaked lesions on leaves. The lesions develop into spots that are varying shades of tan or brown (see picture ‘B’ on parsley whereas advanced spots on cilantro can be black (see picture ‘A’ on cilantro). The lesions are usually limited by leaf veins and thus have an angular, square, or rectangular appearance, a typical feature of bacterial infection. Lesions tend to be relatively small about 1/8 to 1/4 inch (3–6 mm) in diameter and are visible from both the top and bottom of leaves. Under favorable conditions, free moisture from rain or sprinkler irrigation, leaf spots may coalesce and cause considerable blighting of the entire foliage.
Pseudomonas syringae pv. apii (Psa) and P. syringae pv. coriandricola (Psc). cause bacterial leaf spot on parsley and cilantro. Pseudomonas syringae pv. apii (Psa) can cause leaf blight in celery and fennel as well. Though the problem is documented as more of a problem in cilantro and less in celery, in severe condition the disease can result in unmarketable produce in any host. The bacteria are likely seedborne in both crops. However, water from rain, sprinkler irrigation, and heavy dews and fogs will splash bacteria from infected plants onto adjacent healthy foliage resulting in heavy infestation.
To manage the disease, always use tested/treated seeds, rotate crop with non-host to reduce inoculum level, switch from sprinkler to furrow irrigation to limit secondary spread, avoid excessive use of nitrogen fertilizer. Copper spray/copper based fungicide provide limited control against the pathogens.
Mark C. Siemens
Vol. 12, Issue 9, Published 5/5/2021
Automated thinning machines have been commercially available since 2012. These machines identify crop plants and intermittently deliver an herbicidal spray or dose of liquid fertilizer to thin the stand to the desired plant spacing. Some growers have converted older machines to spot apply pesticides to crop plants rather than thin lettuce. Spot spraying just the crop plant makes sense – it reduces applied chemical amount by about 1/3rd as compared to band spraying and by roughly 90% as compared to broadcast. I have heard reports of improved efficacy with this technique, perhaps due to better coverage, however this potential benefit has not been validated in formal trials.
A drawback with automated thinning machines is their high cost. Retail prices for machines are approximately $25,000 per seed line, or about $200,000 for a 4-row, 2-line machine. Another option might be to use automated systems designed for spot spraying weeds. These devices have been commercially available since the mid 90’s and function similarly to automated thinning machines in that they use optical sensors to detect plants and solenoid activated spray assemblies to intermittingly spray unwanted plants (Fig. 1). The cost of these devices is quite reasonable – about $3,000 per unit, or about $24,000 for a 4-row, 2-line machine.
Automated spot sprayers are typically used in agriculture to control weeds in fallow fields (Fig. 2), but could easily be adapted to apply pesticides or even fertilizer to vegetable crops. Spot applying foliar fertilizers to vegetable crops is an interesting concept and is being investigated in California with lettuce.
Another potential use of spot sprayers is to control herbicide resistant weeds. The device can be positioned between crop rows to spot spray a non-selective herbicide to target weeds. Placing the sprayer in a hooded enclosure prevents unwanted drift onto crop plants. We are conducting trials using this technique in cotton this season (Fig. 3). We are also looking for collaborators interested in trying the device as a pesticide and/or fertilizer spot applicator in vegetable crops for this upcoming season. If you are interested collaborating or would like to see a demo of the device, please feel free to reach out to me.
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.