To study symptomology and to find resistant genotypes against Pokkah boeng disease of sugarcane

Introduction

Sugarcane (Saccharum officinarum) is the major C 4 cash crop, having significant economic value and it is widely grown commercial crop grown around the world. It belongs to grass family Poaceae and the genus Saccharum comes under the tribe Andropogon which is mainly found in tropical and sub-tropical regions. The symptoms of Pokkah boeng in Sugarcane mainly occurs in four stages chlorotic-I and II, Top rot and Knife cut symptom (Vishwakarma et al., 2013) [8] . The very first symptom of Pokkah boeng disease is chlorosis towards the base of young leaves followed by twisting and wrinkling and malformation of leaves. Top rot or acute phase is the serious symptoms in which whole base of the spindle gets rotten and it forms a top-rot of the tender tissues of the apical part of the cane. Knife cut phase is also observed in association with the acute phase characterised by transverse cut in the rind of the stalk, as if the tissues are removed with the sharp knife. A study on major (N, P and K) and minor nutrients (S,Zn,Fe,Cu, and Mg) on sugarcane plant affected by Pokkah boeng disease revealed a declined trend in nutrient level within the stalk and leaves of diseased plant as compared to healthy plant. The disease incidence predominantly observed in broad leaves sugarcane varieties particularly in monsoon season potentially leading to reductions in cane weight, internode length, juice percent, girth, pol percent, and total sugars in juice in infected varieties such as CoS 8436 and CoS 88230 (Singh et al., 2006) [7] . The mycelium of Fusarium moniliformae is observed to be thick and cottony texture and the powdery appearance is due to production of macroconidia. The In-vitro studies revealed that Fusarium species cultures form colonies that are woolly, cottony, flat or spreading in appearance (Mui-Yun, 2003) [4] . From the above the colony displays white, cream, cinnamon, salmon, yellow, red, violet, pink or purple meanwhile, the underside of the colonies may exhibit shades such as tan, red, dark purple, brown, or may even appear colourless. Numerous compounds produced by Fusarium species have been reported as harmful or potentially harmful compounds or secondary metabolite with deoxynivalenol (DON) and its derivatives (Munkvold, 2017) [5] .

Results and Discussion

Symptoms of Pokkah boeng disease

Naturally field symptoms and sign of Pokkah boeng disease were observed on different samples across various growth stages of sugarcane under field conditions and the symptoms typically appear mid growth stage i.e. 3 to 7 month of crop. The disease is most prominently observed in the whorl region of young, actively growing plants and progresses rapidly under favourable environmental conditions such as high humidity and warm temperatures. The other favourable conditions for the Pokkah boeng disease include lack of drenched conditions of soil, existence of susceptible varieties and constant cultivation of same varieties. The fungus Fusarium species complex produces various kinds of secondary metabolites called mycotoxins which can easily enter humans and animals through food and feed because of their resistance to milling, processing and heating (Marasas et al., 1991) [2] . The nature and severity of symptoms may vary depending on the sugarcane variety, crop stage and environmental factors. Accurate identification of these symptoms and sign is essential not only for field diagnosis but also for implementing timely control measures to minimize yield loss and disease spread.

Mild Symptoms

In the mild form, the disease was characterized by early signs such as chlorotic (yellowish) streaks and stripes on the younger leaves, slight wrinkling, twisting and curling of spindle leaf blades and leaf tearing with varying intensities. The symptoms under this category include:

A. Chlorosis or whitish streaks and stripes

The first noticeable symptoms of infection often include pale yellow or whitish linear streaks begin to appear along the midrib or margins of the newly emerged leaves. These are often first visible symptoms and are caused by localized fungal invasion disrupting chlorophyll production.

B. Twisting and curling of leaves

Infected sugarcane plants often exhibit twisting, curling, or crinkling of the leaf blades, particularly in the younger leaves. These deformities are attributed to uneven cell growth triggered by the production of fungal toxins, which interfere with normal leaf development.

C. Tearing of leaves

The leaf blades of infected sugarcane plants tear longitudinally, often along the midrib or veins. These tears are irregular and can be mistaken for physical/mechanical damage, but they are actually due to fungal weakening of the leaf tissues. The fungal pathogens produces toxins and enzymes that weakens the leaf tissue which makes the leaf fragile and brittle which leads to split or tear due to wind or natural unfolding.

Moderate Symptoms

When the disease progresses and environmental conditions remain favourable (especially high humidity), the symptoms become more severe and affect not just the leaves but also the central shoot.

A. Top Rot

One of the hallmark symptoms of moderate Pokkah boeng infection is top rot, which results from the pathogen invading and destroying the growing point or shoot apex of the plant. The infection leads to necrosis of the terminal tissues, causing the top portion of the plant to appear rotten, soft and water-soaked. This observation is attributed to the spread of infection from the leaf to the stalk through the growing point, which is consistent with the findings of Martin et al., (1961) [3] and Raid and Lentini (1991) [6] .

B. Reddening of leaf:

Another symptom occasionally observed at the moderate stage is the reddening of leaf blades, particularly in response to prolonged stress caused by fungal infection. This reddening is considered a secondary symptom, often arising due to nutrient imbalances, physiological disruptions or vascular impairment induced by the pathogen. The reddish discoloration may appear as streaks or blotches, especially near the leaf margins or midribs and is more prominent when the plant is under abiotic stress in addition to biotic infection.

Severe Symptoms

The severe stage of Pokkah boeng disease represents the most destructive phase of infection, predominantly observed in highly susceptible sugarcane varieties or untreated fields with prolonged exposure to favourable environmental conditions, such as high humidity, warm temperatures, and dense canopy cover. At this stage, the pathogen has extensively colonized both external and internal plant tissues, leading to profound physiological and structural damage. The resulting symptoms not only compromise plant health but also contribute to significant economic losses due to reduced cane weight, stunted growth, and poor juice quality. The key symptoms associated with this stage are as follows:

A. Yellowing and wilting of open stalk

Yellowing and wilting of the sugarcane open stalk is a symptom that occurs during the moderate to severe stages of Pokkah boeng, especially when the pathogen reaches internal tissues. The apical portion of the cane stalk (the shoot where leaves emerge) begins to turn yellowish. The discoloration starts at the youngest internodes, usually just below the spindle. Yellowing may progress downward, affecting a few upper internodes. This stage reflects a serious disruption of water and nutrient transport within the cane.

B. Knife cut Symptom

The knife cut symptom is the advanced stage associated with top rot phase. At this stage, one or more transverse cuts appear on the rind of the stalk, as if the stem tissue was sliced off cleanly by a sharp knife. This symptom significantly weakens the structural integrity of the cane, making it prone to lodging or breaking under field conditions.

Signs of Pokkah boeng disease

Pokkah boeng disease in sugarcane is characterized by distinct signs, among which whitish mycelial growth is one of the most notable. This whitish appearance results from the development of cottony, thread-like structures called hyphae, which are produced by the fungal pathogen responsible for the disease. These hyphae form the vegetative part of the mycelium, which actively colonizes the plant tissues. The fungal mycelium penetrates the sugarcane's epidermal and cortical regions, facilitating systemic infection. This colonization disrupts normal physiological functions of the plant, leading to tissue softening, distortion and necrosis. In advanced stages, this mycelial growth may be visible on infected plant surfaces, especially in humid conditions and is often accompanied by a foul odour and tissue discoloration. The presence of such fungal signs is critical for early detection and accurate diagnosis of the disease, which is essential for timely management interventions. 23005 and CoM 23006 are found to be susceptible and Co 99004 are found to be highly susceptible. Thus, the resistant varieties can be recommended for farmers and also for the scientific community for improving the breeding programme. Similar results were observed by Wang et al., (2017) [9] and Viswakarma et al., (2013) [8] , they screened twenty two sugarcane genotypes. Out of this, varieties BO139, CoP9301, Co 1148, BO91 etc. were found to be resistant, BO156, BO155, CoP11438 were observed moderately susceptible and the varieties Co 1148, BO130 were found susceptible to Pokkah boeng disease.

References

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