Disease And Insects Take Center Stage

foliar damage from fusarium oxysporum
Foliar damage is one symptom of Fusarium oxysporum race 4, a virulent disease that attacks cotton.


By Bob Hutmacher
Extension Specialist/Agronomist
University of California

We had what seems like another “unusual” spring this year, with a warm mid-March that encouraged early plantings followed by cool and even cold weather on multiple occasions during April and into early May. Even though soils often weren’t wet during this period and rain was somewhat limited, the cooler weather took a toll on many fields, with above-average losses due to seedling disease and slow, delayed leaf expansion.

Drying winds and some hot weather in parts of late April and into May put some added stress on struggling plants. Although we have many fields that are in OK shape in terms of growth at this time, the “legacy” of the starting conditions this year also produced many plants with damaged tap roots, lateral roots trying to get established in drying soils, and some terminal damage that will result in more vegetative branching and fruiting delays.

In our late May and mid-June observations, about two-thirds of San Joaquin Valley cotton fields were five-seven days behind what we think is a more normal rate of development and progression toward first bloom. And perhaps half of the remaining fields are closer to 10-14 days behind typical development for this time of year.

Disease Report
A fair number of fields have been checked out this year for multiple seedling diseases, plus Fusarium oxysporum race 4. Both Rhizoctonia and Thielaviopsis were worse than normal in some fields where plants also grew slowly due to thrips injury. Stand losses were moderate in most cases, even though growth was delayed in many fields. Fusarium oxysporum race 4 continues to expand into additional fields and is worth identifying to try to keep this disease in check.

Prior to first bloom you should consider: (1) investigating areas where plants die off early season; (2) look for root vascular staining to differentiate between Fusarium oxysporum race 4 and other fungal seedling diseases; (3) check with your pest control adviser or university farm adviser if you want a plant/field evaluation; (4) map or take GPS coordinates of areas where there is a known infestation, and plant resistant cotton varieties there in the future.

Troublesome Insect Pests
Lygus populations in many parts of the San Joaquin Valley rose to potential damaging levels starting in late May and going into June this year. This caused some serious early season square losses and required multiple early insecticide applications. Aphid “hot spots” and elevated populations were also observed in multiple locations.

This type of start for the flowering and fruit-production season makes crop management potentially more challenging, producing fruit loss and in some cases terminal damage, altered growth patterns (more vegetative branching, stronger tendency for rapid vegetative growth), and delays in the start of successful fruit retention.

Early use of multiple pesticides can make it difficult to: (1) try to protect beneficial insects; (2) make good choices for pesticides that represent different modes of action for resistance management; and (3) made good choices for effective control of earlier season pests (lygus, aphids, others) that still leave you options for late-season pests … if they show up.

Addressing Fruit Loss
Delayed plant development can also lead to flowering/fruit development shifting even more into what is typically our warmest time of the year. Hot weather and hot nights can negatively affect fruit retention and give more tendencies for strong vegetative growth.

In fields where fruit loss is significant and occurs over an extended period, both plant growth regulator use and irrigation delays may be especially needed management tools this year.

Contact Bob Hutmacher at 559-884-2411 or rbhutmacher@ucdavis.edu.

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