Jun 15, 2022Whiteflies Building Up, Adults on the MoveTo contact John Palumbo go to: jpalumbo@ag.Arizona.edu
Botrytis rot is not considered a major problem in lettuce but it can cause significant damage/loss when the field conditions are favorable for the pathogen. Cool wet conditions are favorable for the pathogen. Symptoms include water-soaked, brownish-gray to brownish-orange, soft wet rot that occurs on the oldest leaves in contact with the soil. Old leaves are more susceptible than young leaves and the fungus can move into the healthy parts. Fuzzy gray growth can be observed in the disease area which is characteristic of the pathogen. In worse cases, the entire plant can collapse. Romaine cultivars, transplanted lettuce that are big and have leaves touching the soil are more susceptible.
The pathogen: Botrytis cinerea
Botrytis cinerea affects most vegetable and fruit crops, as well as a large number of shrubs, trees, flowers, and weeds. Outdoors Botrytis overwinters in the soil as mycelium on plant debris, and as black, hard, flat or irregular sclerotia in the soil and plant debris, or mixed with seed. The fungus is spread by anything that moves soil or plant debris, or transports sclerotia. The fungus requires free moisture (wet surfaces) for germination, and cool 60 to 77 F, damp weather with little wind for optimal infection, growth, sporulation, and spore release. Botrytis is also active at low temperatures, and can cause problems on vegetables stored for weeks or months at temperatures ranging from 32 to 50. Infection rarely occurs at temperatures above 77 F. Once infection occurs, the fungus grows over a range of 32 to 96 F.
Masses of microscopic conidia (asexual spores) are produced on the surface of colonized tissues in tiny grape-like clusters (see picture). They are carried by humid air currents, splashing water, tools, and clothing, to healthy plants where they initiate new infections. Conidia usually do not penetrate living tissue directly, but rather infect through wounds, or by first colonizing dead tissues (old flower petals, dying foliage, etc.) then growing into the living parts of the plant.
1. Buy high-quality seed of recommended varieties. Treat the seed before planting.
2. Practice clean cultivation. Plant in a light, well-drained, well-prepared, fertile seedbed at the time recommended for your area. If feasible, sterilize the seedbed soil before planting, preferably with heat. Steam all soil used for plantbeds at 180 F (81 C) for 30 minutes or 160 F (71 C) for one hour.
3. Avoid heavy soils, heavy seeding, overcrowding, poor air circulation, planting too deep, over-fertilizing (especially with nitrogen), and wet mulches.
4. Focus on healthy plant vigor. Do not over fertilize.
5. Use drop or furrow irrigation instead of sprinklers. If sprinklers have to be used, irrigate morning or early afternoon giving enough time for foliage to dry.
6. Apply recommended fungicides when conditions favor disease development. Make sure to rotate fungicide to avoid development of resistance.
Last fall, we established trials investigating the use of band-steam to control weeds and Fusarium wilt of lettuce in iceberg and romaine lettuce. Band-steam is where, prior to planting, steam is injected in narrow bands, centered on the seedline to raise soil temperatures to levels sufficient to kill weed seed and soilborne pathogens (>140 °F for > 20 minutes). After the soil cools (<1 day), the crop is planted into the strips of disinfested soil.
In the study, we utilized the prototype band-steam applicator (Fig. 1) described in a previous UA Veg IPM articles (Vol. 11 (13) to inject steam into the soil as beds are shaped. The steam applicator was configured to treat a 4” wide by 3” deep band of soil. Three rates of steam (Low, Standard, High) were applied by varying travel speed. The “Standard” rate was where steam was applied at rates needed to reach the target soil temperatures (>140 °F for > 20 minutes). Higher and lower applications rate were examined to ensure target temperatures were met/exceeded to get a better understanding of the efficacy of steam treatment, and to determine if higher travel speeds (less fuel consumption) could be used and still provide effective pest control.
Results showed that application of steam was highly effective at controlling weeds (nettleleaf goosefoot predominant species). At the Standard application rate, over 80% of the weeds were controlled. At High application rates, weed control approached 100%. What was particularly encouraging was that at the Low steam application rate where travel speeds were 60% faster than Standard, and target temperatures were not met, weed control was still very good – about 75%.
Steam treatment was also effective at controlling Fusarium wilt of lettuce. Disease incidence in iceberg and romaine lettuce were reduced by more than 50% as compared to the untreated control (Table 2). Crop plants were noticeably larger and more vigorous throughout the growing season in all steam treated plots (Fig. 2). At the Standard and High application rates of steam, this translated into significant yield increases in iceberg (>300%) and romaine (>90%) lettuce. Significant yield increases were also found at the Low application rate of steam – iceberg (>200%) and romaine (>60%).
The results are very promising, but it is important to note that steam treatment is not an end-all cure for Fusarium wilt disease. At the trial site, disease inoculum levels were considered moderate. However, when inoculum levels are very high, our trials have shown that a 4” wide by 3” band of soil is not effective at controlling the disease. We hypothesize that a wider and/or deeper band of treated soil is needed for effective control. This fall, we will be initiating trials to examine this. We will also be investigating the use of band steam to control pythium and nematodes in carrot. Trial results will be presented in future articles - so Stay Tuned.
As always, if you are interested in seeing the machine operate or would like to test the machine on your farm, please feel free to contact me.
This work is supported by the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council. We greatly appreciate their support. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
Fig. 1. Band-steam applicator principally comprising a 35 BHP steam generator mounted on a bed-shaper applicator sled.
Fig. 2. Iceberg lettuce planted in beds treated with steam (left) prior to planting and untreated (right).
Prefar (bensulide) is an organophosphate herbicide that has been used for more than 50 years in Arizona for lettuce production. In fact, it is one of the standard herbicides used for this purpose.
The University of Arizona Vegetable IPM Team conducted a Yuma County and Imperial Valley Survey evaluating a portion of the acres checked by Pest Control Advisors. The 2017 data indicated that in 58% of the lettuce acreage reported was treated with Prefar. Similarly, in the 2018 survey 52% received a Prefar application.
This product performs well when incorporated with sprinkler irrigation at stand establishment. It also works best in course textured soil’s such as the ones found in Coachella Valley, California. One of the weeds controlled by Prefar is pigweed (Amaranthus palmeri), which was found to be resistant to glyphosate herbicide in Arizona by Dr. William B. McCloskey in 2012. Other weeds are purslane (Portulaca oleracea), goosefoot (Chenopodium murale), Lambquarter (Chenopodium album), and some species of grasses.
The list of crops in which bensulide is used demonstrates the importance of this weed control tool for the agricultural industry.
According to the 1080 Pesticide Use Reporting Database provided by the Arizona Pest Management Center the list of crops includes: Arugula, Bok choy, Broccoli, Brussel Sprouts, Cabbage, Cantaloupe, Cauliflower, Celery, Cilantro, Corn, Cress, Endive, Fennel, Mustard, Kale, Lettuce, Onion, Parsley, Squash, Swiss chard, and others.
Please read the following contribution from Dr. Al Fournier on the EPA Notice- Petition to Revoke Organophosphate Tolerances: (including Bensulide).
Al Fournier, IPM Program Manager, Arizona Pest Management Center
10 August 2022
The EPA has extended its deadline for public comment on a Petition to Revoke Tolerances and Cancel Registrations for Certain Organophosphate Uses until September 25, 2022.
The petitioners, including United Farm Workers, Earthjustice, and several other groups, request that the Agency revoke all tolerances and cancel all associated registrations for food uses of 15 listed OPs, and further requests that the Agency complete its registration review process for these chemicals no later than October 1, 2022. The pesticides are currently at various stages of review, and the proposed deadline does not align with EPA’s published schedule to complete scientific reviews.
Among 15 Organophosphates named in the petition are 10 with reported uses in Arizona (bolded below). Those with known uses in lettuce and other produce include Bensulide (Prefar) and Acephate (Orthene).
Acephate, Bensulide, Chlorethoxyfos, Chlorpyrifos-methyl, Diazinon, Dichlorvos, Dicrotophos, Dimethoate, Ethoprop, Malathion, Naled, Phorate, Phosmet, Terbufos, Tribufos
Read the full EPA Notice here: https://www.regulations.gov/document/EPA-HQ-OPP-2022-0490-0001
To submit comments to the docket, use the following link: https://www.regulations.gov/commenton/EPA-HQ-OPP-2022-0490-0001
To contribute to Arizona Pest Management Center comments, contact Al Fournier