May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
Clovers can be very difficult to control weeds here, but it is also a major crop and common ornamental. Clovers can survive under poor growing conditions and are not controlled with glyphosate and seem to get worse every year. There are more than 50 types and 300 species of clover and they can be easily misidentified. They are all in the legume (Fabracea) family and can use a bacterium (rhizobium) in the soil to convert nitrogen in the atmosphere to a form that they and other plants can use for fertilizer. There are only 4 or 5 clover species that are agricultural pests here. The ones we get the most questions on are white and yellow sweet clover. These are in the Melilotus family. White sweet clover (Melilotus albus) is tall for a clover and can get 3 to 5 foot in height. The leaves are thinner than most clovers and this difficult to control weed lives at least 2 years and sometimes longer. Glyphosate and most of the contact herbicides do not control it. The plant growth regulator herbicides work best. Yellow sweet clover (Melilotus officinalis) is less common here. The flowers are yellow, and it is not as tall and vegetative as white sweet clover. Yellow is more common at higher elevations. California burclover (Medicago polymorpha) and Black medic (Medicago lupina) are in the same genus as alfalfa and are more of a problem in landscapes, parks and golf courses than in agricultural fields here. They do not grow upright and spread below the crop or turf. The true clovers are in the Trifolium genus and include white and strawberry clover. These creep along the ground and root at the nodes of the stem. These are more of a urban landscape weed and not considered an agricultural problem. Creeping woodsorrel or Oxyalis looks like a clover but it is not related. It is a turf weed that spreads rapidly along the ground and can live for several years. Preemergent herbicides are effective against all these clovers before they become established. The postemergence herbicides that are most effective in controlling these clovers are the plant growth regulators. Contact herbicides and glyphosate are generally ineffective.
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
Growers and PCAs can monitor data from the Yuma Leaf Wetness Network through the AZMET website located at the following URL: http://22.214.171.124:460
The website updates information on leaf wetness and near-surface air temperature every 15 minutes. Wetness data are provided in graphical format (see figure below). Output from the leaf wetness sensors increase from the grey (dry) zone of the graph to the blue (wet) zone when wetness (dew or rain) is detected by the sensors.