Pink root is a fungal disease caused by Phoma terrestris, which wreaks havoc on onion plants, destroying their root systems and resulting in leaf wilting and stunted growth. The disease is readily identified by the namesake pink roots exhibited by infected onions. The disease can spread to a variety of other crops, including garlic, cucurbits, corn, soybeans, and more. However, these plants sustain little damage, and primarily serve as reservoirs for the disease.
Pink root’s stunting of overall growth in infected onions is hard to miss, but the telltale pink roots are the easiest symptom by which to initially diagnose the disease.
The wilting of leaf tips is the first symptom to appear after infection, regardless of whether the plant is a seedling or more established. After this, leaves will begin to die back prior to reaching full maturity. As the growing season progresses, the stunted growth of infected plants becomes increasingly obvious, especially when such plants are adjacent to healthy plants. Infected plants generally look similar to plants which are experiencing drought conditions or are severely nutritionally deficient.
All the symptoms apparent aboveground are a product of the disease’s advances belowground. In the early stages of infection, infected roots turn yellow, then transition to pink, red, purple, and finally black, before shriveling and dying. Newly produced roots typically die quite quickly.
When a badly infected plant is uprooted, most of the roots will be dead or entirely rotted away, with the remainder showing the disease’s namesake discoloration. In severe cases of pink root, onion bulbs will be stunted, and the scales may exhibit the same pink or red discoloration as the roots, and may be rotted as well. Even bulbs which appear healthy enough to be harvested may rot in storage.
The disease rarely kills more established plants, though seedlings may die. The primary economic consequences of pink root result from the bulbs of infected plants being rendered unsellable due to stunting, softening of tissues, or rot. Symptoms are worst in plants which are infected in the spring, as their root systems are too badly damaged to keep up with water demands as summertime temperatures rise.
Pink root is caused by Phoma terrestris, a soilborne fungus found throughout the United States.
P. terrestris is an opportunistic fungus which can survive in a vast variety of plants. In particular, its ability to readily reproduce in corn—where it causes the disease red root rot—means that the fungus is essentially ubiquitous in all major agricultural regions of the United States.
The fungus becomes most active during periods when temperatures are between 75- and 85-degrees Fahrenheit, and onions grown in fields that are poorly drained are especially vulnerable. However, P. terrestris’ affinity for warm weather should not be mistaken as evidence of fragility. The fungus can easily survive for years in soil, feeding on plant debris where available.
When onions or other viable hosts are planted, fungi inoculum migrate to the root surfaces, where they release enzymes which break down tissues and allow the fungi to gain entrance. The rate and severity of infection is often magnified in fields where other pathogens, such as Pythium, have already gained a foothold and have rendered plants more vulnerable.
Eliminating pink root requires mitigation practices incorporating crop rotation, irrigation improvements, and proper nutrition.
Pink root can be an extremely frustrating disease to manage. For every year that onions are grown in infected fields, without interruption, inoculum levels in the soil are magnified. Consequently, the symptoms of the disease will become increasingly severe every successive season.
This is why crop rotation is a key means of reducing disease inoculum. Onions should be rotated out of infected fields for a period of at least 3 to 6 years. If possible, avoid growing other crops which can be host to P. terrestris. But this is quite difficult, given the range of hosts which the fungus can infect.
However, plants of the genus Allium (such as onion, garlic, scallion, leek, shallot, and chive) must be avoided, as these are particularly hospitable, and thus result in increased inoculum levels in cultivated soils. Corn and cereals must also be avoided.
If crops being grown are of a high enough value, it may be economically feasible to fumigate soils. If so, metam sodium and chloropicrin have both been shown to be effective in some cases. However, solarization is a more effective and economic means of reducing inoculum levels in the short term.
Ensuring improved irrigation of soils is also critical in controlling pink root. Not only are the symptoms of the disease more severe in such conditions, but the fungus has been shown to spread through surface water. This means that improving drainage in fields can control the severity and spread of pink root.
There are some varieties on the market which have been claimed to demonstrate resistance to pink root. While this may be the case in ideal conditions, we have long observed that such costly resistant varieties often become vulnerable to disease if they are grown in substandard conditions.
Thus, the more cost-effective approach is to implement balanced nutritional regimes which ensure adequate soil levels of boron and calcium, both of which are critical to the growth of hardy, disease-resistant tissues, as well as improved immune response. Incorporating products such as Fusion 360 Soil and Iota 0-0-1 will also increase microbial activity and diversity, which increases competition and serves to naturally reduce P. terrestris inoculum, as well as the inoculum of other harmful pathogens.