The Re-Emergence Of Hoja Blanca Disease In The United States

FELIPE DALLA LANA AND MADISON FLASCO

BATON ROUGE, LOUISIANA

The list of pathogens causing disease in Louisiana’s rice fields is long, and it may soon grow due tothe re-emergence of a viral disease that has not been seen in the state for at least 60 years. Hoja Blanca disease, Spanish for "white leaf," is a new threat to the U.S. rice industry. The disease is caused by rice hoja blanca virus and is transmitted by the rice delphacid planthopper (Tagosodes orizicolus) vector, a topic discussed in another article in this newsletter. This article will briefly describe the disease's history, symptoms, and potential damage, as well as identification, control methods, and the current status of the disease in Louisiana. 

Historic and Geographic Distribution 

Hoja Blanca was first documented in Colombia in 1935. Since then, it has spread across countries in South and Central America. In the United States, 

the vector and virus were first reported in Florida in 1957, followed by Mississippi in 1958 and Louisiana in 1959. The disease has not been observed in the United States since the mid-1960s. However, the disease remains a significant threat in South and Central America, causing substantial yield loss during outbreaks. 

Symptoms and Damage 

The symptoms of Hoja Blanca become visible 5 to 30 days after infection by the planthopper. The classic symptoms include chlorotic (yellowing) to completely bleached leaves, for which the disease is named. 

The severity of symptoms depends on the growth stage at which the plant is infected. When young plants are infected, they exhibit chlorotic and bleached foliar symptoms and may also be stunted, have reduced root growth, and produce stunted or deformed tillers. These foliar lesions can eventually become necrotic, and the plant may die. 

When the infection occurs later in the plant's development, the leaf symptoms will still be present, but stunting is not observed. The panicle of an infected plant will be partially or completely sterile and is often partially retained in the boot. Additionally, the hulls can be distorted and discolored. Seeds from infected plants perform poorly in the field and are frequently more susceptible to other diseases. Infected, mature plants are often symptomless. Yield reduction can range from 25% to 75%, but complete loss has been documented in susceptible varieties. The severity of symptoms can also vary amongst cultivars. 

The disease cycle 

The rice delphacid can acquire the virus after feeding on an infected plant for 1 to 12 hours and will retain the virus for the rest of its lifespan. After acquisition, the virus multiplies inside the insect's body. It can take 15 to 25 days for the virus to reach the delphacid’s salivary glands, at which point the insect is able to transmit the virus to a healthy plant. Transmission from an infected insect to a healthy plant occurs after 3 to 7 hours of feeding. 

The progeny of an infected delphacid is also born infected and can transmit the virus sooner than those that acquire it through feeding. Progeny from infected delphacids is likely responsible for the rapid spread of the disease during outbreaks. 

The virus also has detrimental effects on the delphacid. Infected insects often experience respiratory and organ failure, ultimately reducing their lifespan. The reproductive process is similarly impacted. Males become less fertile, females lay about one-third as many eggs as non-infected females, and the number of nymphs that reach adulthood is reduced. Under field conditions, only 5-15% of captured delphacids are infected with the virus, although this number can reach 35% during an outbreak. 

Management

Managing plant viruses relies on cultural practices, primarily the use of resistant and tolerant varieties. In Central and South American breeding programs, some levels of resistance to the vector and the virus have been identified. The literature indicates that tropical japonica rice varieties are more resistant to the virus than indica varieties. Chemical control of the vector can be implemented, but care should be taken to ensure the chemistries preserve populations of natural enemies and can be used in conjunction with crawfish production. 

The current scenario for hoja blanca in Louisiana 

As mentioned, Hoja Blanca has not been officially reported in Louisiana in recent years. However, considering the widespread presence of the vector in Louisiana, Texas and Arkansas, and the virus being in all production areas of Texas, it is plausible that the virus could already be present in some fields where the vector is found. 

The re-emergence of Hoja Blanca and the delphacid presents several challenges for disease management. Our current rice production system could potentially increase the risk of this disease becoming endemic to the region. The widespread adoption of second-crop rice and crawfish production creates the opportunity for rice plants to be in the field year-round. Particularly troubling is the cultural shift of rice production from the disease’s first appearing in the states to now. It is estimated that less than 10,000 acres of crawfish were produced in Louisiana in the mid- 1960s; today, this number is close to 370,000 acres according to the 2024 Louisiana Ag Summary. The extended growing period of rice and crawfish production provides ample host plants for both the virus and the delphacid vector across multiple growing seasons. Further complicating the spread of the virus and vector are extreme weather events, such as hurricanes and tropical storms that can potentially spread the delphacid across large geographical regions. 

Most of our knowledge on this system is based on research done during the relatively short epidemics in the late 1950s and early 1960s or in other countries, particularly Colombia. There is a lot to learn and validate for our current conditions and management practices. Currently, the Rice Pathology Program from the Rice Research Station and the Plant Virology Laboratory from the Department of Plant Pathology are working on a number of initiatives to improve our capacity to respond to the disease. As the virus is not yet detected in the state, there are limitations on the research that can be done. However, work is underway in collaboration with other universities and international institutions, especially the CIAT in Colombia, to improve our response to this new threat. 

We encourage growers to contact extension agents or the Rice Research Station if they see what appears to be Hoja Blanca disease in their fields. The first step to combating this threat is to understand its presence.   ∆

FELIPE DALLA LANA AND MADISON FLASCO

LSU AGCENTER