It is that time of year again when temperatures are rising, melon plantings are maturing, and sudden wilt and death of melon plants is occurring. What causes this sudden collapse of plants? In the desert melon production areas of Arizona and California, symptoms of plant wilting and collapse usually can be attributed to one of four diseases; Charcoal rot, Fusarium wilt, Monosporascus root rot, or Pythium sudden wilt. Each disease is caused by a different soil-borne plant pathogen, so knowing what management options might be available first requires accurate identification of the responsible pathogen. Charcoal rot and Fusarium wilt, caused respectively by the fungal pathogens Macrophomina phaseolina and Fusarium oxysporum, are not effectively controlled by fungicides. Preventative actions that may reduce the severity of these diseases include planting resistant melon varieties when available (for Fusarium wilt) and minimizing plant stress. Plant stress due to over- or under-irrigation can be managed; however, other crop stress factors such as fruit load and hot temperatures are obviously beyond the grower’s control. Monosporascus vine decline, caused by Monosporascus cannonballus, can be suppressed by application of the fungicide Cannonball at seeding or transplanting followed by additional applications as specified on the product label. The other disease mentioned was Pythium sudden wilt. Pythium, the pathogen that causes this disease, is a fungus-like soil-borne organism that can be managed by fungicides, such as mefenoxam. However, the difficulty in preventing extensive Pythium sudden wilt is that once disease symptoms begin to appear and the pathogen is identified, rapid deployment of an effective fungicide treatment will protect noninfected plants but probably not save plants already infected but not yet displaying sudden wilt symptoms. Knowing the melon disease history in a particular field can help immensely in preparing for possible recurrence of diseases in a future melon planting caused by a soilborne plant pathogen.

Have questions about plant disease issues? Beginning today, I will have dedicated office hours from 1:00 to 5:00 p.m. every other Wednesday (on the day that this newsletter is published). Come in to discuss a particular plant problem or plant diseases and their management in general. Of course, if you need plant disease diagnosis services outside of these hours, you can still bring in specimens to the lab anytime from Monday through Friday, 8:00 a.m, to 5:00 p.m.
Dr. Mike Matheron, University of Arizona Yuma Agricultural Center, 6425 W. Eighth Street, Yuma, AZ 85364. Cell phone: 928-941-2094. Email: matheron@ag.arizona.edu.

To contact Mike Matheron go to: matheron@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.