Mosaic discoloration of leaves on a squash plant

Every growing season, farmers prepare for the expected diseases. You plant at the appropriate time, buy varieties that have resistance or tolerance to various diseases, utilize fungicides that have protective properties to try to reduce the chance of developing problems, water in a timely manner, and grow your crop with a multitude of best management practices. One of the most frustrating things you may encounter, however, is something that is difficult to prepare for, the onset of a virus.

Unlike a bacteria or fungus, there are no chemical preventives or controls for viruses.  Seeds can be infected with a virus, or a virus can be introduced through a wound in the plant. A virus can affect a small portion of the plant through cell to cell movement, or a virus can become systemic and travel throughout the vascular system of the plant and wreak havoc.

Symptoms of systemic viral infection include the following:
  • Malformations:  stunting, twisting  of the growing tips,  leaf curling, leaf distortion,
  • Epinasty:  the unequal growth of two surfaces leading to curling of the whole leaf
  • Enations:  outgrowths from upper or  lower surfaces of leaves
  • Delayed senescence (plants remain vegetative)
  • Mosaics-patterns of light and dark areas or streaks or stripes
  • Stunting and Dwarfing
  • Death of plant tissue (necrosis)
  • Mottling or chlorosis of plant tissue

*Source of information: Dr. Jane Polston, Viruses that Infect Plants, UF Plant Pathologist

When symptoms first begin to develop in a plant, your first instinct may be that there is a nutrient deficiency.  If a grower is questioning whether it is nutrients, disease, or virus, please contact your County Extension Agriculture Agent to help make a determination.  Often agents have to send a representative plant sample to the UF-NFREC Plant Pathology lab for diagnostics, but the lab will run a barrage of tests to determine the cause of the problem.

Malformation of the leaves and early discoloration

Mottling or chlorosis of plant tissue

Recently in Escambia County, a squash plant that was showing unusual symptoms was submitted to the pathology lab for evaluation.  Although further tests are ongoing, the sample tested positive for the Potyvirus group.  It tested negative for a number of virus problems, including the following commonly occurring viruses on squash: Cucumber Mosaic Virus, Papaya Ringspot Virus (formerly Watermelon Mosaic Virus I), Squash Mosaic Virus, Watermelon Mosaic Virus, and Zucchini Yellow Mosaic Virus). Potyviruses are aphid-transmitted and sometimes seed-transmitted.

What should a farmer do?  There are no controls for viruses. Since this is a potyvirus, reducing the chance of aphid transmission is important.  Using reflective mulch may help prevent aphid transmission of viruses. In a greenhouse setting, removing and discarding infected plant material, and controlling the aphids will greatly help control the disease.


Additional information about squash viruses

Recommendations for Management of Whiteflies, Whitefly-Transmitted Viruses, and Insecticide Resistance for Production of Cucurbit Crops in Florida

2017 Florida Plant Disease Management Guide: Squash