May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
When known weedy fields are ready to plant and labor is expected to be short, it is tempting to use all the preplant herbicides that are available. In lettuce, there are three preplant herbicides available and it is not uncommon to use 2 and occasionally all 3 on the same crop. All three of these herbicides use the same mode of action to kill weeds. There are slight differences between them but they all either stop or disrupt cell division in the roots and or stems of the weeds. They are normally safe to lettuce unless the crop is stressed or the rate, timing or placement are poor. The rationale for using multiple preplant herbicides in lettuce is often to broaden the weed control spectrum or guard against misses caused by misapplication or environmental conditions. There are some hazards, however, that sometimes outweigh the benefits. Potential crop injury is increased. All 3 use the same mode of action and the chance of injuring developing crop roots is compounded. Sometimes herbicides are added that contribute nothing but potential injury to the mix. If you look at the following chart you can see that many weeds are controlled by Kerb, for instance, that are not controlled by Balan or Prefar. Why add them? All three control grasses, goosefoot and purslane. If environmental conditions and applications are optimal it is often possible to use only one. Herbicides are much less expensive than labor, but it is possible to overdo it and cause more problems and expense.
With the start of melon season, you will also start seeing seedling diseases on melon. Bacterial fruit blotch (BFB) of melon is a common disease in melon seedlings that can stay in the crop until fruits/seeds are formed. You will be seeing more of this disease in greenhouse transplants.
Bacterial fruit blotch (BFB) of watermelon is caused by the bacterium Acidovorax avenae subsp. Citrulli. The bacteria produceslarge olive green to brown water-soaked lesions on fruit, making them unmarketable.
Symptoms of BFB on seedlings begin with water-soaked areas on the lower surface of the cotyledons and inconspicuous lesions on leaves. BFB lesions will become necrotic often with yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die.
Greenhouse conditions are usually favorable for dispersal and establishment of pathogen. Thus, good greenhouse practices and sanitation is extremely important. Clean transplant trays must be used (disinfect trays if they will be reused) and new soil. Destroy any volunteer seedlings and keep the area in and around the greenhouse weed free. Avoid overhead watering if at all possible, or water in the middle of the day so that the plants dry thoroughly before evening. The bacterium can spread on mist and aerosols. Relative humidity should be kept low through proper watering and good air circulation in the greenhouse. Separate different seedlots, to reduce lot-to-lot spread. Monitor these isolated seedlings daily and destroy trays where symptoms develop. The remaining trays should be sprayed with a labeled bactericide and the applications continued until the plants are transplanted to the field.
The pathogen can be seedborne, so growers should only use seed that has been tested for the presence of the pathogen by a reputable testing facility. Management of BFB includes a combination of preventing the introduction of the pathogen, sanitation to eliminate any inoculum present, and the use of bactericides if the disease appears. There are no commercially available watermelon cultivars that are resistant to bacterial fruit blotch, but there is some variation in susceptibility among cultivars.
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://18.104.22.168:460
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.