Symptoms and Control Methods of Watermelon Green Mottle Mosaic Virus

In recent years, in most areas of Liaoning with Gaixian as the center, the majority of watermelon growers have faced a headache: just when the harvest was about to be secured, they found that watermelons which looked fine on the outside had deteriorated flesh inside——showing a "frozen heart flesh" and "spoiled flesh" appearance, greatly reducing their commercial value, and farmers have suffered immensely. "Why do the watermelons I grow develop such a severe 'frozen heart flesh disease'? What kind of disease is it exactly? How can I manage it so they don't get sick?" Such questions also once gave us, as a seed business department, a headache. We have always wanted to solve this problem well for farmers so that they can obtain high returns. After consulting many parties, with the help of the provincial and municipal plant protection departments, we finally found the answer and gained a deep understanding of this disease. We now introduce the situation of this disease to the broad community of farmers. We hope it can help farmers solve this headache.
The scientific name of "frozen heart flesh disease" is watermelon green mottle mosaic virus, and farmers commonly call it "frozen heart flesh" because of its appearance. The pathogen of this disease is Cucumber green mottle mosaic virus (CGMMV—W cucumber green mottle mosaic virus), which belongs to the plant virus disease category.
Watermelon green mottle mosaic virus is mainly transmitted through seeds (watermelon and rootstock seeds), soil, plant sap, the grafting process and agricultural operations.

1. Seed transmission
The transmission rate of watermelon seeds harvested from diseased plants is about 1%, and the transmission rate of the gourd rootstock used for grafting is 1%—6%. This shows that if virus-carrying seeds are used, the probability of infection will be quite high. If suitable conditions occur, initial infection can take place, and the virus on the seed coat can be transmitted to the cotyledons, and after 21 days it can cause symptoms to appear on young tender leaves.

2. Soil transmission
Infection occurs in continuously cropped melon fields. The virus overwinters and survives in the remains of diseased plants. In soil it still retains pathogenicity for 1 year and 2 months, becoming a source of infection the following year. The soil virus-carrying rate is 0——3.5%.

3. Transmission through agricultural operations
Although the seed transmission rate is relatively low compared with the field incidence rate, it is very easily transmitted through field contact, such as through grafting, transplanting, vine training, intertillage and other agricultural operations; if there are strong winds and heavy rain in the field, causing plants to collide with each other and branches and leaves to rub together, the virus sap can then spread through wind and rain, and the disease is more severe in a high-temperature, high-humidity environment. It is likewise severe when temperatures are high in greenhouses.
Now that we know the conditions for the onset of this disease and its transmission routes, how do we judge its symptoms? Watermelon green mottle mosaic virus mainly damages leaves, fruit stalks and fruits.
(1) Infection at the seedling stage: irregular chlorosis (green mottling) and brown mosaic appear on new leaves. When the disease is severe, the green parts of the leaves bulge and the leaf surface becomes uneven, and the leaves become narrower and smaller. At the seedling stage, the virus has already completed the infection process of its entire life cycle and shows relatively obvious symptoms, making it easy to detect. (This is also the most important stage for control.)
(2) Growth stage: in infected plants, vine growth is poor and the plants do not grow luxuriantly. Mosaic symptoms are not obvious on old leaves and mature leaves, but compared with healthy leaves they are mostly slightly yellow. (At this time the symptoms are not obvious and are not easy to detect.)