May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
It is much easier to kill weeds when there is no crop in the field and now is a good time to reduce the seed bank of summer annual weeds in fallow fields. Weed seeds are buried at variable depths in the soil, some have hard seed coats and there are other variables that cause them to germinate over a long period of time. If they all came up at the same time they would be much easier to control. It takes time, therefore, to repeatedly irrigate, germinate and kill weeds with either tillage or herbicides. We have conducted trials that indicate that in most years summer annual weeds begin to germinate in February, reach a peak in June but continue to germinate into October.
Proper timing of tillage to kill weeds can be important with some species. Some weeds like common Purslane are very succulent and can remain viable for several days after cultivation or hoeing. They can reroot at the nodes and continue to grow if they are allowed to get too big before they are uprooted. Growers sometimes allow early emerging weeds to get fairly big in an effort to germinate as many seeds as possible. Incorporating large amounts of organic matter into the soil can also have a negative effect on some preemergent herbicides used in vegetables. Many of the root and shoot inhibitor herbicides like Trifluaralin, Pendimethalin, Benefin, DCPA and others can bind to organic matter and be less available to kill weeds.
Tillage has the opposite effect on perennial weeds such as nutsedge and bermudagrass than it has on annual weeds. These weeds are spread vegetatively and repeatedly irrigating and tilling them will spread rather than kill them.
Both contact and systemic herbicides are used during fallow periods to control weeds. The contact herbicides include Paraquat (Gramoxone, Firestorm), Carfentrazone (Aim, Shark), Pyraflufen (ET), Pelegonic Acid (Scythe),Glufosinate (Rely,Liberty) and others. Some of the advantages of these are that they are quick and have no soil residual allowing crops to be planted soon after application. Disadvantages are that they are effective primarily only on small weeds.
The most commonly used systemic herbicide for fallow ground is Glyphosate. It is broad spectrum and has no soil residual. Many of the systemic herbicides registered for fallow use, such as Oxyfluorfen (Goal, Galigan) or EPTC (Eptam) require at least 90 days before planting many vegetable crops. If done correctly, Eptam can be very effective in controlling nutsedge during summer fallow.
Only the fumigants kill weed seeds. These include Chloropicrin, Methyl Bromide, Metam Sodium, Dazomet, Telone and others. Most preemergent herbicides only work after the seed has germinated. Preemergent herbicides are often used for fallow weed control only when at least 30 to 45 days or longer are available. Fumigants are expensive, can be difficult to use and are often used for disease or nematode control with the added benefit of controlling weeds. Unlike soil active herbicides, Fumigants do not have any residual activity.
Soil solarization and flooding have become increasingly popular in recent years as techniques to control pests during summer fallow. Few regions are as well suited for these techniques as the low desert. They are used primarily to control diseases but have the benefit of controlling some summer annual weeds as well. Summer flooding works better here in the low desert than it does in many places because of the high temperatures and high respiration demands. The availability of oxygen is cut off to the roots when it is most needed. It is necessary to keep the field continuously flooded at a depth of 6 to 8 inches for 3 to 8 weeks. Some species are much more sensitive than others to this technique. Perennial weeds are more sensitive than are many annual weeds. Pigweed, field bindweed and nutsedge survive while many annual grasses do not.
Gearing up for the produce season, we already have been seeing plenty of fields transplanted with cole crops (cauliflower, cabbage, broccoli etc). Below are the few common diseases to watch out for.
This disease commonly affects seeds and young transplants and is caused by the soil-borne fungi such as Pythium, Fusarium, Rhizoctonia etc. Infected seeds decay in the soil. Seedlings and young transplants will “damp-off” or rot at the soil line, before they eventually collapse and die.
The fungus, Rhizoctonia solani, causes wirestem. Stems of plants become constricted and brittle at the soil line. The plant becomes stunted and may rot at the soil line. This disease is more severe on fall cole crops when the soil is warm. We have seen lot of this problem in the fields last year. Make sure you get certified disease free seedlings.
Prevention & Treatment: Cultural controls include planting on raised beds and providing good drainage. In greenhouse where transplants are grown, use new potting soil and new or thoroughly cleaned and disinfested containers and trays. Wash used containers with soapy water to remove all traces of old soil mix, and then briefly submerse containers in a 10% bleach solution. Allow to dry before planting in containers. Both in greenhouse and fields: avoid overwatering and wet feet in plants/seedlings.
Black rot is another common disease we observed in the fields last growing season. Black rot is caused by a bacterium, Xanthomonas campestris pathovar campestris, and can affect all vegetables in the crucifer family. Above-ground parts of the plant are primarily affected, and symptoms may vary depending on the type of plant, age of the plant and the environmental conditions. In general, yellow, V-shaped lesions appear along the tips of the leaves with the point of the V directed toward a vein. When lesions enlarge, wilted tissue expands toward the base of the leaves. Veins turn black or brown. Infection may spread into the stems. Cutting into the stems often reveals a black-brown discoloration with a yellowish slime present. Symptoms on cauliflower may appear as numerous black or brown specks, black veins and discolored curds.
Prevention & Treatment: With no effective curative measures available, preventative measures are very important. The bacteria survive the winter on plant debris and on weeds, such as wild mustard and Shepherd’s purse. It also can survive in and on seeds from infected plants. It can remain alive on plant residue buried in the soil for up to two years. The disease is easily spread by splashing water, wind, insects and garden tools. High temperatures and humidity favor development of the disease.
Use certified disease-free seed and transplants. If source of the seeds is unknown, or infested seedlots must be used, treat seed with hot water to eradicate pathogenic bacteria. Cabbage, broccoli, and Brussels sprouts can be treated at 122 °F for 25 minutes, while seeds of cauliflower, kale, turnip, and rutabaga are treated for 15 minutes. However, this treatment may reduce the viability of seed.
Choose varieties tolerant to black rot. Do not plant cole crops where black rot has occurred in the past two to three years. Select well-drained sites with good air circulation.
This disease is caused by the fungus Peronospora parasitica and can attack both seedlings and mature vegetable plants. Infected plants develop a gray mold on the lower leaf surface. The upper leaf surface of infected plants first turns yellow and then may turn brown or necrotic. Leaves wither and die. Symptoms differ from powdery mildew in that the downy mildew fungus grows only on the lower surface of the leaf. Development of the disease is favored by moist conditions.
Prevention & Treatment: Use varieties with resistant varieties. Rotate with crops other than cole crops or greens. Remove plant debris immediately after harvest. Use wide plant spacing to promote drying of leaves. For chemical control, make sure to rotate the fungicides to avoid development of resistance.
Alternaria Leaf Spot
Alternaria leaf sport is a common problem and sometimes may not be of economic importance. However, if the plants are already weak or physically damaged providing the site of infection, the disease can cause economic losses.
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://22.214.171.124:460
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.